Clean up & standardise TypeScript conventions

[jovyntls]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
As we've been doing quite a large-scale file-by-file migration to TypeScript (#1913), there is some inconsistency and redundancy in how we handle some types, exports, etc. With #2201 and #2221, we now have a more organised typing system and a rough "convention" that can be followed by TS files.

This PR:

• standardises class exports
• standardises how we represent generic key-value types to use Record
  • Record<K,V> is interchangeable with { [key: K]: V } syntax, but is more extensible, generalisable, and is an officially supported utility type in TS.
• fixes some typos in our docs, as many js files are now ts files
• removes unnecessary as unknown as and any typecasts, now that we have a more mature typing system

Anything you'd like to highlight/discuss:
N/A

Testing instructions:
N/A

Proposed commit message: (wrap lines at 72 characters)

Clean up & standardise TypeScript conventions

The core package is almost fully migrated to TypeScript. There are
several inconsistencies and redundancies in how we represent common
types, handle typecasts, and export/import files. These inconsistencies
may cause confusion, reduce code quality, and prevent us from fully
taking advantage of static typing. 

As of #2201 and #2221, we have a more mature typing system that we can
utilise to fix these inconsistencies as well as a general set of
conventions that can be followed across TS files. 

Let's,
- Standardise class exports
- Standardise key-value types to use `Record<K, V>`
- Remove unnecessary `as unknown as` typecasts
- Reduce the use of `any` types
- Fix outdated docs that refer to `.js` files

Using Record is preferable as `Record<K,V>` is interchangeable with
`{ [key: K]: V }` syntax, but is more extensible and arguably more
readable. `unknown` and `any` types also introduce uncertainty and
unsafe typecasts into the codebase.

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[ong6]

LGTM

[tlylt]

I briefly looked through and looks good to me. Curiously most of the changes in the form of{ [key: string]: string }; to Record<string, string>; will make the type work better or? What's the advantage of Record

[jovyntls]

• Record<K,V> is interchangeable with { [key: K]: V } syntax, but is more extensible, generalisable, and is an officially supported utility type in TS.

@tlylt

• both Record<K,V> and { [key: K]: V } are currently used in our codebase
• Record<K,V> is interchangeable with { [key: K]: V } syntax
• Record<K,V> is more extensible, and is an officially supported utility type in TS
• Record<K,V> is more flexible, and can support more types for the key (K). It supports generic types which is not supported in { [key: K]: V }.
  • e.g. if we have type ColorName = "red" | "blue"; we can do Record<ColorName, ColorVal> but not { [key: ColorName]: ColorVal }