Taxonomies: Custom metabox UI

[JoryHogeveen]

https://developer.wordpress.org/reference/functions/register_taxonomy/
See meta_box_cb and meta_box_sanitize_cb.

Currently we only support the default display types for taxonomies.
I've seen requests/questions about this in multiple occasions where people want to allow only one taxonomy or just a different UI type.

With the pick field UI types and the DFV add-new options we could provide various UI types, compatible with the blocks editor, to select/manage taxonomies attached to posts.

[jimtrue]

As long as this gets us away from people using relationships for taxonomy, I'd suggest this as a good thing.