Missing icons for H5 and H6?

[markadrake]

While customizing the Tiptap editor today I noticed that we do not have icons for H5 and H6.

Using the following:

icon-heading-1
icon-heading-2
icon-heading-3
icon-heading-4
icon-heading-5 (*)
icon-heading-6 (*)

(*) Supplying these values to the uui-icon element result in an empty space.