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/video/accessible_menu.scss

@@ -1,3 +1,5 @@
+@import 'base/mixins';
+
 $a11y--gray: rgb(127, 127, 127);
 $a11y--blue: rgb(0, 159, 230);
 $a11y--gray-d1: shade($gray, 20%);

xmodule/css/video/display.scss

@@ -8,7 +8,12 @@
 // --------
 // Defaults: what displays if the icon font doesn't load.
 // --------
+@import 'vendor/bi-app/bi-app-ltr';
 @import 'bourbon/bourbon';
+@import 'lms/theme/variables';
+@import 'bootstrap/scss/variables';
+@import 'bootstrap/scss/mixins/breakpoints';
+@import 'lms/theme/variables-v1';
 
 // the html target is necessary for xblocks and xmodules, but works across the board