Handle MSC3952-style mentions with the WYSIWYG editor

[clokep]

As part of #24377 initial support for MSC3952 was implemented in matrix-org/matrix-react-sdk#9983. This did not include support for the WYSIWYG editor (only the current editor) since that's currently undergoing significant work.

Generally this should be handled near where we attach relations, see here.

[daniellekirkwood]

@jakewb-b FYI 👀