Nimble spinner spec (#837)

# Pull Request

## 🤨 Rationale

Creating spec for spinner component.
Also see #346

## 👩‍💻 Implementation

Based on 'custom component' template. Deriving from the FAST
`progress-ring` is an option but not currently planned for this spinner
(see spec).

## 🧪 Testing

N/A

## ✅ Checklist

- [x] I have updated the project documentation to reflect my changes or
determined no changes are needed.

change/@ni-nimble-components-cb67d3ce-526f-411d-ba8d-60cea6651936.json

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+{
+  "type": "none",
+  "comment": "Add spec document for nimble-spinner.",
+  "packageName": "@ni/nimble-components",
+  "email": "20709258+msmithNI@users.noreply.github.com",
+  "dependentChangeType": "none"
+}

packages/nimble-components/src/spinner/specs/README.md

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+# Nimble Spinner
+
+## Overview
+
+Spinner component (`nimble-spinner`): Indeterminate progress indicator / loading indicator. Meant to be temporarily used in a region of a web page until that region's contents have loaded, or to represent an ongoing operation that doesn't have a known/ determinate duration.
+
+### Background
+
+[Nimble spinner: #346](https://github.com/ni/nimble/issues/346)  
+[Nimble spinner design: #822](https://github.com/ni/nimble/issues/822)  
+[Visual Design spec - Adobe XD](https://xd.adobe.com/view/33ffad4a-eb2c-4241-b8c5-ebfff1faf6f6-66ac/screen/dece308f-79e7-48ec-ab41-011f3376b49b/)
+
+[Follow-up issue about PowerGreen/custom color variants: #916](https://github.com/ni/nimble/issues/916)
+
+### Non-goals
+
+Ability to show determinate progress:
+
+-   The current visual design for the spinner is distinct/different from an indeterminate radial progress bar
+-   We don't currently have visual designs for a linear or radial progress bar with determinate progress
+-   If/when we want progress bar components in the future, at that point we'll probably create new Nimble components which extend the corresponding FAST components:
+    -   `nimble-progress` or `nimble-linear-progress`, deriving from `fast-progress`, for a linear progress bar; and
+    -   `nimble-progress-ring` or `nimble-radial-progress`, deriving from `fast-progress-ring`, for a radial progress bar
+
+### Features
+
+-   Show indeterminate progress with an animation
+    -   Cannot be paused - assumption is that component will be added to/ removed from the page as needed
+-   Supports 3 sizes: `small` (16x16), `medium` (32x32) (default), `large` (64x64)
+    -   Component will scale to other sizes if needed (if width/height styles are set on it directly), for advanced use cases
+
+### Risks and Challenges
+
+None
+
+### Prior Art/Examples
+
+SystemLink Enterprise uses a spinner based on the Material `mat-spinner`:  
+![SystemLink Material spinner](SlMatSpinner.gif)
+
+A variant of this spinner design is already implemented in the Web Config tool written in Blazor:  
+![Blazor Web Config spinner](BlazorWebConfigSpinner.gif)
+
+## Design
+
+The `nimble-spinner` is an animating indicator that can be placed in a particular region of a page to represent loading progress, or an ongoing operation, of an indeterminate / unknown duration.
+
+### API
+
+Component Name: `nimble-spinner`
+
+Properties/Attributes:
+
+-   `size`
+    -   String attribute controlling size of the component
+    -   Backed by an enum `SpinnerSize` with values `small` (16x16), `medium` (32x32), `large` (64x64)
+    -   If omitted, the default is `medium` (32x32)
+    -   If omitted, custom `width` and `height` styles can be set on the `nimble-spinner` and the animating elements will scale to that size. In this case, consumer should follow up with Design to doublecheck the padding needed.
+
+Methods, Events, CSS Classes, CSS Custom Properties: None
+
+### Anatomy
+
+Contains 2 `div` elements that are animated via CSS animations.
+
+Slot Names, Host Classes, Slotted Content/ Slotted Classes, CSS Parts: None
+
+### Angular integration
+
+Directive `NimbleSpinnerDirective` targeting selector `nimble-spinner`, with bindings for `size`.
+
+### Blazor integration
+
+Standard Blazor implementation (`NimbleSpinner` deriving from `ComponentBase`), with bindings for `size`.
+
+### Visual Appearance
+
+[Visual Design spec - Adobe XD](https://xd.adobe.com/view/33ffad4a-eb2c-4241-b8c5-ebfff1faf6f6-66ac/screen/dece308f-79e7-48ec-ab41-011f3376b49b/)
+
+![New spinner design](NewSpinnerDesign.gif)  
+**Note:** 'Color UI Alt.' appearance is unsupported for the initial implementation, see the _Potential Future Enhancements_ section.
+
+When `prefers-reduced-motion` is enabled, a simplified version will be used:  
+![Spinner, Prefers Reduced Motion](SpinnerPrefersReducedMotion.gif)
+
+## Implementation
+
+Component implementation will derive from the FAST `FoundationElement`.
+
+**Alternatives Considered:** We could derive from `fast-progress-ring` (radial progress indicator, which supports an indeterminate progress display), however then our component API would also pick up the `min`/`max`/`value` properties for determinate progress (and `paused` to pause the indicator), which we don't plan on supporting at this time. Additionally, the visual appearance of our spinner will be a little different from a radial progress bar, and we still may end up supporting a standard radial progress bar in the future.  
+For reference, the FAST spec for `progress` / `progress-ring` is [here](https://github.com/microsoft/fast/blob/802443ffb2b19a078f9b48f62e6d1a35e3276fb5/packages/web-components/fast-foundation/src/progress/README.md).
+
+### States
+
+Only one component state: visible and animating. Pausing not supported; spinner is expected to be added/removed from the DOM as needed.
+
+### Accessibility
+
+No keyboard states / form integration.
+
+ARIA: We plan to use the [`progressbar role`](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/progressbar_role) without setting `aria-valuemin` / `aria-valuemax` / `aria-valuenow` (representing indeterminate).  
+This matches the behavior of an indeterminate FAST progress ring (in which case min/max/value are unset) - see the [FAST template here](https://github.com/microsoft/fast/blob/802443ffb2b19a078f9b48f62e6d1a35e3276fb5/packages/web-components/fast-foundation/src/progress-ring/progress-ring.template.ts#L17).
+
+`prefers-reduced-motion`: A simplified spinner indicator will be shown when this is enabled, see the Visual Appearance section.
+
+### Globalization
+
+N/A.
+
+RTL: No special handling / no changes to the spinner animation. Linear indeterminate progress animations are sometimes reversed for RTL, but circular/clockwise ones are usually not (see [Material Design v2, Mirroring Elements](https://m2.material.io/design/usability/bidirectionality.html#mirroring-elements) > _When Not To Mirror_, for one example).
+
+### Security
+
+N/A
+
+### Performance
+
+N/A
+
+### Dependencies
+
+None
+
+### Test Plan
+
+Standard tests planned.  
+[Chromatic is supposed to handle pausing CSS animations by default](https://www.chromatic.com/docs/animations), however we should verify that by ensuring the snapshots of the spinner show the same state across 2 Chromatic builds.
+
+### Tooling
+
+N/A
+
+### Documentation
+
+N/A - Standard updates of the component table in the README, and in Storybook.
+
+## Potential Future Enhancements
+
+-   Do we need to support the "Color UI Alt" (PowerGreen) spinner appearance, and do apps/consumers need a way to override the spinner color?
+    -   Original plan for supporting that appearance: adding a `theme-variant` attribute:
+        -   default (undefined): Standard appearance (light/dark/color themes)
+        -   `prominent`: Alternate appearance/color, for the color theme only ('Color UI Alt.' from XD design)
+        -   Backed by enum `SpinnerThemeVariant`
+    -   We could also follow the patterns from our icons (create a token `spinner-color` that can be overridden)
+    -   **Decision**: We will omit the Color UI Alt appearance for our initial implementation. Followup questions that need to be answered are captured in [Issue #916](https://github.com/ni/nimble/issues/916).