Org mode's tag completion commands all use a custom completion function,
which makes it difficult for alternative completion functions to support
well.

Emacs already has a function for reading multiple things,
completing-read-multiple, which can be used for the tag completion use
case.

I have attached a patch for this change, which I have tested manually a
few times and also fixed the existing tests.  I have tested this with
vertico, which claims to strictly follow Emacs's completion API.