Migrate CKEditor 5 to TypeScript MVP

[arkflpc]

It's not feasible to reimplement all the code-base of CKEditor5 in TypeScript in one go. We should start with something that meets the main goals of #11704 and let us build upon that.

Proposed scope:

• rewrite some packages to TypeScript,
• make those packages tested by automated tests (tests can stay in JS),
• make those packages testable by manual tests,
• configure linter rules for TS,
• configure DLL builds,
• automatically generate documentation from TS files in the same way as from JS files (i.e. JsDoc),
• generate npm package, so it can be used in backward-compatible manner (if possible),
• include typings in npm package

The packages to rewrite should be:

• utils (because it's the bottom of the whole dependency chain, and it doesn't change often)
• engine
• core
• essentials plugins (as a model how to consume engine & utils types)

We should aim for statically-typed editor config that covers every other plugin in CKEditor5 (and be extendable by custom plugins if possible)

ToC

• [TS MVP] Generate docs from old *.js files #12078
• [TS MVP] Update NPM release process #11720
• [TS MVP] Update DLL build process #11718
• [TS MVP] Research how to build API Docs from *.ts files #11721
• [TS MVP] Update ESLint configuration #11719
• [TS MVP] Manual tests should be able to cover code written in TypeScript #11717
• [TS MVP] Automated tests should be able to cover code written in TypeScript #11716
• [TS MVP] Rewrite ckeditor5-utils package to TypeScript #11715
• [TS MVP] Enable 'translations:collect' for typescript #12029
• [TS] Decide how to generate type definitions & enable testing them #11722
• [TS MVP] Rewrite ckeditor5-ui #11726
• [TS MVP] Rewrite ckeditor5-core #11727
• [TS MVP] Create typings for editor config #11728
• [TS] Revise code style guide to cover TypeScript #11723
• [TS MVP] Rewrite ckeditor5-engine #11724

[arkflpc]

The rationale for including essential plugins is: if we actually use the basic API (utils, engine, core) in plugins, we can find out that some types have to be rewritten to meet real-world use cases. So we'd be far from declaring it stable.