feat(vi): support deleting chars backward with X

[sim590]

Summary

Add support for the X command in vi normal mode, which deletes the character before the cursor and places it in the cut buffer, mirroring the behavior of X in vim.

This partially addresses #621.

Changes

• Parse X as Command::DeleteCharBackward in the vi command parser
• In normal mode, emit MoveLeft + CutChar to delete the character before the cursor and place it in the cut buffer
• In visual mode, emit CutSelection (consistent with x behavior)
• Add parser tests for X and 2X

Known limitation

When used with a multiplier (e.g. 3X), each character is cut individually, so only the last deleted character remains in the cut buffer. This is a pre-existing limitation shared with x and will be addressed in a follow-up PR.