feat(vi): add u, U visual mode commands

[matystroia]

This PR adds the following commands to visual mode:

• u to convert selection to lowercase
• U to convert selection to uppercase
• ~ to switch case for each character in selection

These match default vim mappings and would be especially useful when working with environment variables.

Added:

• Command::Lowercase
• Command::Uppercase
• EditCommand::LowercaseSelection
• EditCommand::UppercaseSelection
• EditCommand::SwitchcaseSelection

[kronberger-droid]

Thanks for this.
Parser logic and mode guards look right and u/U/~ in visual mode go through cleanly, and u → Undo in normal mode is properly gated.

One pre-existing limitation this surfaces: vi/mod.rs:86-89 consults normal_keybindings in both Normal and Visual modes.
Users with bare u rebound to Undo will see undo fire in Visual instead of Lowercase.
The parser's mode gates are correct, but the keybinding lookup runs first.
I will have to make a follow-up PR for adding visual_keybindings to Vi so this doesn't entangle.

Also what through me off while testing is that single grapheme selections don't work, which is a repeating issue for Vi Mode.

But all of these are not your problems to worry about. Fine with landing this now and thinking about the other stuff in follow-ups.

@fdncred are you fine with this?

[fdncred]

Seems reasonable. It's up to you @kronberger-droid

[matystroia]

Also what through me off while testing is that single grapheme selections don't work, which is a repeating issue for Vi Mode.

I noticed this as well since vU is a common pattern for capitalization. Fortunately I think the fix for it is fairly simple! I've created a draft PR for it here: #1089

[kronberger-droid]

Could you still add a couple tests:

• Editor-level coverage for the new EditCommands on a fresh buffer
• End-to-end vU/u, since this is the case that motivated fix(vi): enter visual mode through command #1089

Otherwise good to land once tests are in.