Move to TypeScript

[CvX]

This PR converts the codebase to TypeScript.

Remaining tasks:

• Resolve all “TODO: [TS]” comments
• Add TS type definitions to electron-util, electron-debug, electron-dl, electron-context-menu, facebook-locales
• Fix "build/files" in package.json, try out the build
• Look into TS linting via ESLint/xo
• Enable stricter TS rules in tsconfig.json, fix all errors

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Fixes #740

[CvX]

Any reason why source specifically? Isn't src a convention for TS projects?

That's what I settled on and use in other projects too. src goes back to the old Unix times where file paths were limited. source looks better and is more readable. No good reason to shorten it.

Should we change dist too? 😉

Jokes aside, what about dist-ts? I've named it that for a lack of a better idea at the time.
Should we move that inside dist, e.g. dist/js or is it better to keep those two separate? If so, at least I'd replace "ts" in that name to "js" (since that's what's inside).

[sindresorhus]

Yeah, I usually would go for distribution, but since we already have dist, which we cannot change as it's controlled by electron-builder, I think we should go with dist-js.

[sindresorhus]

@CvX Is there anything left to do? If not, I'll do a final review and merge.

[CvX]

1. Have to merge the master to this branch to fix the conflicts. I'll do it shortly.
2. Ideally we would get facebook-locales definition out of the repo. I've submitted the PR (Add TypeScript definition wix-incubator/facebook-locales#6) but maybe it will go to DefinitelyTyped instead. In any case, we could update that later, after this gets merged into the master.
3. I'm not sure about other .d.ts files (those that are Caprine-specific). They are just thrown out there, I feel like they could probably should be colocated with they code that uses these definitions, but that would have to wait for a bit of a refactoring (where we would group code of a single feature together, like we do with emoji now). Again, not a blocker IMO.

[sindresorhus]

Wooooot!! Super nice work on this. It makes the codebase so much easier to work with. ❤️