Enhancement/issue 426 restore userland postcss

[thescientist13]

Related Issue

resolves (part of) #426

This PR is just focusing on PostCSS support for now. Another PR(s) will come for Babel and Browserslist

Summary of Changes

1. Restored userland PostCSS configuration in Rollup and CSS Tranforms
2. Add projectDirectory to compilation.context for convenience
3. Restored nested CSS for all www/styles/ and www/components/
4. Re-enabled PostCSS test cases
5. Updated Build configuration section of the docs to demonstrate new behavior

Notes / TODOs

1. Update documentation - wait for Docs/issue 430 update technical documentation to reflect current project status #452 to be merged first? (then should be able to use context too in the transform?)
2. Should we consider extending this support to <style> tags too? (as another issue at least) - tracked in Trello for post 1.0 tracking
3. Probably also for another issue, but right now we have to process everything twice to support the user's PostCSS (once in serve and again in bundle / rollup since we read from file, not the server) . It would be nice if Rollup could start from the processed output of the serve lifecycle so we don't have to re-run userland PostCSS again. - added to Roadmap To 1.0 And Beyond #418
4. Noticed that the CSS puppeteer (and whitespace from us?) puts into an index.html page is almost 10KB!!! This may be causing style collisions colliding with styles 👆 ? Either way, we should control the HTML output. If want to support inlining CSS great, but then we also shouldn't ship it over the network either. One or the other. Will make an issue to track this specific issue of junk in the trunk, and another to track optimizing for inlining CSS. - added to Roadmap To 1.0 And Beyond #418

view-source:https://deploy-preview-436--elastic-blackwell-3aef44.netlify.app/

After deleting all that....