Add "CSI u" extended keyboard mapping

[christianparpart]

trafficstars

See http://www.leonerd.org.uk/hacks/fixterms/

I wonder why the spec didn't make it runtime configurable, such as SM/RM to explicitly request that.

Should be configurable via contour config in profiles.*.extended_keybboard_mapping: BOOL or so.

https://sw.kovidgoyal.net/kitty/keyboard-protocol/ seems to be an updated one.

[WSLUser]

Iterm2 has this option available as a setting you can toggle on and off. Maybe seeing their implementation may help.

[christianparpart]

Correcting myself against CSI u and towards modifyOtherKeys from xterm

• https://invisible-island.net/xterm/manpage/xterm.html#VT100-Widget-Resources:modifyOtherKeys
• https://invisible-island.net/xterm/modified-keys.html

[christianparpart]

CSI u is implemented on master branch.