This issue is a collection of improvements for the Template editor introduced with #6541:

### Ideas
- [x] add plugin interface to let plugins provide different editors/previews (to let plugins provide e.g. drag and drop editors)
- [ ] easily upload report assest via drag and drop in code editor
- [ ] add ability to edit report snippets (maybe also the ability to switch between them using tabs)
- [ ] add custom template filters api form field component to visually edit template filters
- [ ] codemirror infer available django context variables
- [x] add vertical resize drag handler
- [x] ~~add printcss preview type (https://printcss.live/)~~=> can be a plugin now
- [ ] define a custom language for django templates to be more accurate (example: https://github.com/codemirror/lang-liquid)
- [ ] ??? maybe switch to monaco ???
- [ ] allow previewing without saving the template (ref: https://github.com/inventree/InvenTree/issues/5841#issuecomment-1963014776)
- [ ] show warning if there are unsaved changes before closing the drawer