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

xmodule/css/annotatable/display.scss

@@ -4,6 +4,11 @@
  * that the LMS did, so the quick fix was to localize the LMS variables not shared by the CMS.
  * -Abarrett and Vshnayder
  */
+@import 'bourbon/bourbon';
+@import 'lms/theme/variables';
+@import 'bootstrap/scss/variables';
+@import 'lms/theme/variables-v1';
+
 $annotatable--border-color: $gray-l3;
 $annotatable--body-font-size: em(14);