Incorporate/utilize/support Tiptap v3's new official `FontSize` extension instead of or in addition to mui-tiptap's own `FontSize` extension, if available

[sjdemartini]

https://tiptap.dev/docs/editor/extensions/functionality/fontsize

Remove our own extension in favor of Tiptap's?

This extension is part of the general https://tiptap.dev/docs/editor/extensions/functionality/text-style-kit