Allow custom `delete-region`, `insert` functions

[45mg]

Make it possible to specify which functions are used to delete the region and insert text.

This is needed to get Meow to work with modes like Vterm, that define their own delete-region and insert functions, without massive code duplication.

See meow-vterm for an example of this - it requires this patch to work.

[45mg]

Force-push to define functions meow--insert and meow--delete-region, which should be less annoying than having to (funcall meow--X-function) every time.

[45mg]

Force-push because the previous one confused funcall and apply. Oops.