Decouple XModule styles from LMS/Studio styles (attempt 3) (#32237)

This basically changes how the xmodule static files are
generated and consumed in order to separate the Xblock
styles from general style files. Includes:

* build: decople XModule style assets by using a custom webpack loader
* build: move scss imports to its specific file
* build: fix: add system dirs to theme lookup paths.  (fixes attempt 1)
* build: fix: use bootstrap variables instead of lms variables (fixes attempt 2)

This is an amendment to #32188,
which itself was an amendment to #32018.

Addressing the issue https://github.com/openedx/edx-platform/issues/31624

cms/envs/common.py

@@ -1439,7 +1439,8 @@ REQUIRE_DEBUG = False
 WEBPACK_LOADER = {
     'DEFAULT': {
         'BUNDLE_DIR_NAME': 'bundles/',
-        'STATS_FILE': os.path.join(STATIC_ROOT, 'webpack-stats.json')
+        'STATS_FILE': os.path.join(STATIC_ROOT, 'webpack-stats.json'),
+        'LOADER_CLASS': 'xmodule.util.xmodule_django.XModuleWebpackLoader',
     },
     'WORKERS': {
         'BUNDLE_DIR_NAME': 'bundles/',