Migrate to TS, make type definitions available

[birdofpreyru]

And it is blocked, at least, by the need to migrate postcss-modules-parser first, as we need TS types for it (birdofpreyru/postcss-icss#1)

[birdofpreyru]

Well, it looks like the codebase migration from Flow to TypeScript is not possible at the moment:

• It requires quite an effort.
• Not all 3rd party dependencies provide TypeScript types (e.g. there is no @types/css-loader, nor types bundled into css-loader package itself).