InputBase: Add isBorderless prop

[mirka]

Prerequisite for #56524 and possibly #57720

What?

Adds a isBorderless prop to the internal InputBase component, and connects the component to the Context System so it can also be toggled "under the table".

Why?

Some recent design specs require the border to be hidden in certain components (e.g. SearchControl, maybe SelectControl).

We don't quite want to surface this as an official prop on public components themselves because they require careful holistic assessment in terms of accessibility. But it's useful to have as a semi-private prop on an internal building block like InputBase.

Testing Instructions

Apply this diff to test

diff --git a/packages/components/src/input-control/input-base.tsx b/packages/components/src/input-control/input-base.tsx
index ec468ce7fd..bf68458720 100644
--- a/packages/components/src/input-control/input-base.tsx
+++ b/packages/components/src/input-control/input-base.tsx
@@ -77,7 +77,7 @@ export function InputBase(
 		hideLabelFromVision = false,
 		labelPosition,
 		id: idProp,
-		isBorderless = false,
+		isBorderless = true,
 		isFocused = false,
 		label,
 		prefix,

Apply the diff above and see the Storybook for some InputBase-based components, like InputControl or SelectControl.

Screenshots or screencast

This is just an example of what happens when the borderless flag is enabled in an InputBase-based component, in this case SelectControl. Obviously, other design/accessibility considerations need to be taken into account before shipping this in a public component.

[github-actions]

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Core SVN

Core Committers: Use this line as a base for the props when committing in SVN:

Props 0mirka00, mciampini.

GitHub Merge commits

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

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[mirka]

Technically, we can already just use this .components-input-control__backdrop selector to override border styles from any consumer. Still, I think it's better to control this centrally because it's intertwined with the border styles in the focused and disabled states, as you can see in the input-control-styles.tsx file.

[mirka]

This prop shouldn't be exposed in the APIs of any public component.

[mirka]

Unrelated to borderless, but I'd like to sneak in this small enhancement for better themability if that's ok 🥺

[github-actions]

Flaky tests detected in 4f38523.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7805003098
📝 Reported issues:

• [Flaky Test] aria-disabled is correctly hydrated for different values #54370 in /test/e2e/specs/interactivity/directive-bind.spec.ts

[ciampo]

🚀