Add support for OSC-777 notifications

Implement OSC-777, which lets you trigger a system notification.
Already supported by other terminals like kitty, foot, urxvt.

You can easily try this out by running this in your bash:

printf '\x1b]777;notify;Hello;World\x1b\\'

See:
https://chromium.googlesource.com/apps/libapps/+/HEAD/hterm/docs/ControlSequences.md#OSC
https://github.com/kovidgoyal/kitty/blob/9d98aa315922119e8a845046416432548d665b36/kitty/window.py#L811
(cherry picked from commit 6ba3cbb1)