Restore select2 input functionality after Bootstrap 5 cleanup

- Restored select2 input type from earlier commit d4adf5e^
- Added select2 import to bootstrap5-editable.js bundle
- Updated demo with select2 example (requires select2 library)
- Enhanced README.md with comprehensive select2 documentation
- Includes support for static sources, AJAX sources, and advanced configuration
- Select2 library must be included separately (not bundled)

src/bootstrap5-editable.js

@@ -19,6 +19,7 @@ import "./inputs/list.js";
 import "./inputs/text.js";
 import "./inputs/textarea.js";
 import "./inputs/select.js";
+import "./inputs/select2/select2.js";
 
 // Date input (now uses npm bootstrap-datepicker)
 import "./inputs/date/date.js";