[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

Commands executed:
  composer remove scssphp/scssphp
  composer remove scssphp/scssphp -d typo3/sysext/core --no-update
  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/+/81295


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: Oliver Hader <oliver.hader@typo3.org>