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/static/sass/_build-v1.scss

@@ -84,8 +84,6 @@
 
 // +Xmodule
 // ====================
-@import 'xmodule/modules/css/module-styles.scss';
-@import 'xmodule/descriptors/css/module-styles.scss';
 @import 'xmodule/headings';
 @import 'elements/xmodules'; // styling for Studio-specific contexts
 @import 'developer'; // used for any developer-created scss that needs further polish/refactoring