Add a way to create plugins in TypeScript

Right now plugin need to be a function with fn.postcss = 1 marker.

But in TS there is no way to create function and sen key in the same manner. So create plugin is no easy.

I’ll implement a typed one-argument postcss.plugin(creator) helper that marks and returns the creator function, while preserving the legacy two-argument API, and open a focused PR with runtime, type, and documentation coverage.