Add grammar scope to EditorView

Only works when the `grammar-changed` event has been triggered,
and only implemented for Space Pen views.
Use data attribute instead of CSS classes
Make sure the scope data attribute is always set
Implement support for React Editor
Change attribute name to `data-grammar`
:white_check_mark: Add test to verify implementation
:bug: Fix incorrect assertions
Merge pull request #2996 from thomasjo/editorview-grammarscopes

Add grammar scope to EditorView