[BUGFIX] Apply CKEditor5 CSS scoping client-side

Download CSS via fetch() and apply CSS scoping and
URL rebasing on the client in order to avoid the
need for a server side SASS wrapper that:
 * can not relocate relative paths
 * can not easily check if the resource is web accessible
 * had to use regex-replacements instead of straight
   forward replacements on a properly parsed syntax tree

The composer dependency scssphp/scssphp could theoretically
be removed, but is kept in the v12 backport in order to
avoid breaking classic mode extensions that relied on the
fact that this dependency is shipped with TYPO3 v12.

Commands executed:
  cd Build
  npm install --save css-tree
  npm install --save-dev @types/css-tree
  npm run build

Resolves: #101918
Resolves: #100768
Resolves: #101380
Releases: main, 12.4
Change-Id: I34dfd76c07584a482f9012de1c693f0e7f9a2cb8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81308


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>