feat: implement db and cb

[BrettKulp]

Implement db for delete the previous word and cb for delete the previous word and enter insert mode

[leohenon]

Thanks! I just pushed a couple cleanups.

This is fine for now. The current vim handler already hardcodes basic operator combos, so this fits the pattern. Longer term I’d still like to move this toward a proper operator + motion abstraction as part of #5 but thats a different scope.