feat: Repeat operations for add/change/delete.

[kylechui]

TODO: Repeat deletions, unit test stuff.

Feels like there's a cleaner way to parse the user input than the scuffed nil stuff I have right now. Although at the very least it's isolated in the get_char function 🤔

[kylechui]

Another issue that has come up is the idea of deleting multiple surrounds. If we do ds3), but the buffer only has ((hello)), should the delete function no-op or do a "best-effort" and try to delete as many as possible?

[kylechui]

The case (foo) cursor_here (bar) is so cursed with ds2b---I want to avoid mutating the buffer until everything is found so change_surround can highlight everything properly, but finding the next possible surrounding pair to delete while ignoring already found pairs seems incredibly hard, if not impossible. If only I didn't implement reverse search.... 😭

Because I'm selfish I'm going to remove reverse search from the plugin in this branch just to see how the plugin feels