This pull request adds support for upper / lower casing a visual
selection with `u` or `U` respectively, and resolves #6901.

I took an approach to split out the logic of the existing `ChangeCase`
action into something that could be reused for the two new actions. One
area that I could use feedback on is whether or not vim binding updates
make sense. Judging purely from the way things work from a clean install
of Vim / Neovim, I'm pretty sure that it's a bug that `u` applies an
undo in visual mode, so I think it's safe to make this change, but it
changes a behavior that some users may be used to by now in Zed.

Release Notes:

- Added support for `u` and `U` in Vim visual mode
([#6901](https://github.com/zed-industries/zed/issues/6901)).