[CKEditor 5] Update imports for CKEditor 5 v46 compatibility

[Mati365]

Hi 👋

Some imports used in the MathType CKEditor 5 integration are no longer available in CKEditor 5 v46 and above. This is because several export names and paths have changed in the latest release.

UpcastWriter was renamed and is no longer available from its previous path. It’s currently used here:
https://github.com/wiris/html-integrations/blob/3161db7f83633a8d58095a7ea9ca5c1c4624da17/packages/ckeditor5/src/plugin.js#L4C62-L4C74

To keep the integration working, these imports should be updated accordingly. Without these changes, the plugin won’t work with newer versions of CKEditor.

More details:
ckeditor/ckeditor5#18583

Thanks!