emacs: Add as Transpose Characters (editor::Transpose) (#22974)

Originally reported here:
-
https://github.com/zed-industries/zed/issues/4856#issuecomment-2578468329

macOS default vscode keymap already has this:

https://github.com/zed-industries/zed/blob/8d42456b8a7c547c6760b604c24d902bd2e12b8b/assets/keymaps/default-macos.json#L55
But it's disabled on Linux default vscode keymap as VSCode has this bind
instead:

https://github.com/zed-industries/zed/blob/8d42456b8a7c547c6760b604c24d902bd2e12b8b/assets/keymaps/default-linux.json#L407

Explicitly add it to both emacs keymaps so we can keep them identical
between macos/linux as long as possible.

Release Notes:

- emacs: Add support for `ctrl-t` transposing characters on Linux