refactor: remove unused Block/Elem exports from bem.ts

Remove legacy exports that are no longer used after completing BEM migration.

Removed exports:
- Block (component) - 0 usages in source code
- Elem (component) - 0 usages in source code
- BemWithSpecificContext (factory) - 0 usages
- useBEM (hook) - 0 usages
- BemComponent (type) - 0 usages

Kept exports (still in use):
- cn (function) - used in ~99 source files
- BlockContext (context) - used in 2 test files
- CN (type) - return type of cn()
- CNTagName (type) - used in Button.tsx and Taxonomy.tsx

This completes the BEM migration cleanup. All components now use cn() helper.

refactor: remove BlockContext export from bem.ts

Remove BlockContext from exports as it's no longer used anywhere in editor.

After cleaning up test files to remove unnecessary BlockContext.Provider
wrappers, BlockContext has 0 usages in the editor codebase.

Final bem.ts exports (minimal set):
- cn (function) - primary BEM helper used in ~99 files
- CN (type) - return type of cn()
- CNTagName (type) - used for dynamic tag types in Button and Taxonomy

All legacy Block/Elem system exports now fully removed.

Author: luarmr

web/libs/editor/src/utils/bem.ts

@@ -1,11 +1,5 @@
 export {
   cnb as cn,
-  Block,
-  Elem,
-  BemWithSpecificContext,
-  BlockContext,
-  useBEM,
   type CN,
   type CNTagName,
-  type BemComponent,
 } from "@humansignal/core/lib/utils/bem";