Decouple XModule styles from LMS/Studio styles (attempt 2) (#32188)

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. 

This is an amendment to #32018

Addressing the issue #31624

xmodule/css/annotatable/display.scss

@@ -4,6 +4,10 @@
  * 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 'lms/theme/variables-v1';
+
 $annotatable--border-color: $gray-l3;
 $annotatable--body-font-size: em(14);