Extract the core of postcss-lit to allow other css-in-js syntaxes to be built

as per the title, we should extract the core of postcss-lit since it isn't actually specific to lit.

this would allow the community to build other css-in-js syntax support without having to reinvent the wheel.

**this is just an issue to track my progress in doing so**

cc @karlhorky @kutnickclose

related styled-components/styled-components#3897 stylelint/postcss-css-in-js#225