WebUI: Replace html-minifier with postcss+plugin for CSS minification.

 1) Remove html-minifier from third_party/node/package.json.
 2) Replace with postcss (main tool) and postcss-minify (postcss plugin)
 3) Update html_to_wrapper() and css_to_wrapper() GN rules to invoke
    the new tool.
 4) Fix a few newly surfaced CSS syntax errors that were previously
    uncaught, and now surface as build errors.
 5) Update tests to include cases where CSS nesting and container
    queries are used, as well as $i18n{} placeholders to ensure the
    new minifier handles them correctly, unlike the previous one.

With #1 and #2 above node_modules.tar.gz slightly decreases from 9.3MB
to 9.1MB.

The new minifier unblocks using newer CSS features, previously blocked
on html-minifier not parsing them correctly, or forcing client code to
use "clean-css ignore" special comments to work around html-minifier.

Bug: 40943411
Change-Id: I32357c18d94ebe6fe1be0c84a7f21d5e92dff5ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6594216
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1468997}