Add a button to copy all changes as a diff to clipboard in Source Control view

It would be useful to have a button in the Source Control view that copies
all current changes (staged and unstaged) to the clipboard, formatted as a
readable diff grouped by file.

This is helpful when sharing changes outside of git itself, for example
when pasting into a chat, an issue, or a code review tool that doesn't
have direct repository access.

I've already implemented this as `git.copyChanges` in the built-in Git
extension and will open a PR shortly. Opening this issue for visibility
and in case there's feedback on the approach before it's reviewed.