Remove xmodule assets from the edx-platform build

[kdmccormick]

Prior work:

• Remove special XModule asset handling #31624

### Tasks
- [ ] Replace `xmodule_[display|edit]` with standard XBlock CSS classes
- [ ] Load each builtin XBlock's JS as standalone, regular XBlock JS rather than webpacking it into edx-platform
- [x] Convert builtin XBlock Sass into regular XBlock CSS, using design token CSS vars for theming

[kdmccormick]

We we will handle this as part of the XBlock Extraction effort.