[WIP] Adopt TypeScript for backend

[le0tan]

Do NOT review, still working on copying over non-JS files after tsc because of microsoft/TypeScript#30835

What is the purpose of this pull request?

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

Resolves #1421

Overview of changes:

Anything you'd like to highlight / discuss:

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)

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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 blatantly unrelated changes
• Pinged someone for a review!

[ang-zeyu]

In case anyone wants to work on this, this is ⭐up for grabs⭐! (we've discussed in private)

Do NOT review, still working on copying over non-JS files after tsc because of microsoft/ TypeScript#30835

For the main issue raised here, perhaps we can stick with

a.js
a.ts
b.js
b.ts

in the same directory for now, its not uncommon.

As raised in the ts repo issue above, bundling assets is rightly not something in the scope of the typescript compiler.
Introducing webpack / babel toolchain for our cli / core packages seems overkill for a node application as well.

The folders might get a little cluttered but iirc there are a few features on popular ides like webstorm / vscode to "hide" the generated js files anyway (probably only doable once we've completely migrated though).

[tlylt]

Shall we close this PR given the preliminary work done in #1877 ?