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/poll/display.scss

@@ -1,3 +1,8 @@
+@import 'bourbon/bourbon';
+@import 'lms/theme/variables';
+@import 'bootstrap/scss/variables';
+@import 'lms/theme/variables-v1';
+
 div.poll_question {
   @media print {
     display: block;