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

Examine the current postcss-lit codebase to understand how it extracts CSS from Lit template literals. Identify the core parsing and transformation logic that is framework-agnostic. The goal is to refactor this into a reusable library, allowing plugins for other CSS-in-JS syntaxes like styled-components. Check related issues in styled-components and stylelint for context on desired features.

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