Replace post css selector parsing with the official library

[itsdouges]

Right now we're parsing it with a string.split - we should move over to the official parsing library instead: https://github.com/postcss/postcss-selector-parser

Code to change: https://github.com/atlassian-labs/compiled-css-in-js/blob/master/packages/ts-transform/src/utils/css-transform.tsx#L30

Now what we have works - so there's no real pressing need to change it. But this is a good first issue I think 😄.

---

That said looking at some of the issues on there are interesting (like: postcss/postcss-selector-parser#204) - but we probably have the same issue ATM tbh.

[pgmanutd]

Good first issue 😋😋Should I start with this 😉 😉

[itsdouges]

Hi! Yeah sure 😄 Give it a go.