Emacs keybinding improvements (2025-01-02) (#22590)

Various improvements to the emacs compatibility keybindings.

- See also: https://github.com/zed-industries/zed/issues/4856

Release Notes:

- Improvements to emacs keybindings:
- Better support for running emacs inside Zed terminal (e.g. `ctrl-x
ctrl-c` will quit emacs in terminal not zed)
  - `alt-^` Join Lines
  - `ctrl-/` Undo
  - `alt-.` GotoDefinition and `alt-,` GoBack
  - `ctrl-x h` SelectAll
  - `alt-<` / `alt->` Goto End/Beginning of Buffer
  - `ctrl-g` as Menu::cancel