[v4] Expose AST scope and parsing methods

[sonofmagic]

This proposal is aimed at exposing more low-level capabilities of PostCSS, similar to TailwindCSS, to provide plugin developers in the community with better adaptability.

In the current Tailwind CSS functionality, we may need to perform some static analysis on existing projects, including parsing the project's CSS, obtaining the AST, and performing transformations (similar to PostCSS's API).

However, the currently exposed API does not provide such capabilities. Therefore, the purpose of this PR is to introduce these changes.

import { CSS, parseCss, CompileOptions } from 'tailwindcss'

function getContext(css: string, opts: CompileOptions = {}) {
  return parseCss(CSS.parse(css), opts)
}

const ctx = await getContext('@import \'tailwindcss\';')
// do stuff
// ...

PR Link: #16542

There is also another discussion that focuses on plugin development for community developers.
For more details, see: #16546