Add support for u and U in vim visual mode

[bswinnerton]

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).

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[bswinnerton]

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