Migrate project to TypeScript ✨

[Kocal]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Issues	Fix #816
License	MIT

[Kocal]

TIL: packing Encore with pnpm and installing it in another project with pnpm add -D ../path/to/encore.tgz breaks the auto-completion in webpack.config.mjs in PHPStorm (still works in VSCode), when it works with webpack.config.mts in PHPStorm (and VSCode too).
But, installing Encore in a test project with pnpm add -D github:symfony/webpack-encore#typescript-migration works! Surely pnpm is doing something weird that PHPStorm can't resolve, but at least we know.