Vimdiff support

[antonmry]

Following this discussion, this is a feature request to add support for neovim/vim diff with the current CopilotChat diff functionality.

It could work in a similar way to the existing diff mappings (gd, gy, gq, gj) but it would open the diff (assuming COPILOT_GENERATE prompt) in a new vertical buffer and it would execute a 'diffthis` command in the last active buffer and the new vertical buffer.

Some possible variations:

• The Copilot buffer could be reused to load the diff.
• Vimdiff may be only executed in the buffer with the diff, letting the user to execute ":diffthis" in whatever other buffer they choose.

[antonmry]

I implemented a PoC here in case it's helpful: https://github.com/antonmry/diff_copilot.nvim

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