Ship ESM so postcss can run natively in the browser

Right now in order to use post css in the browser

• You have to rely on a bundler and stub fs, path modules
• Use a CDN like jspm which takes care of cjs to esm conversion

It would be great if postcss was to ship ESM directly which could be loaded directly in the browser with importmaps. It seems like most of the code would work without the need of fs module - which could in any case be stubbed the way jspm does.

Ref #830 , #913