feat: expose CSS AST

[mikededo]

Description

Currently, it is not possible to get the css generated tree, as all generated css by the parse function is ignored. AST types are already public, since sveltejs/svelte#12968.

While I don't know the technical reasons on why this was not implemented initially, I think it'd make sense to maintain the CSS provided structure, adapted to ESLint needs.

[mikededo]

I've found that through the context.sourceCode.parserServices, it is possible to get the StyleContext, which should be enough.