feature request: support full CSIu kitty keyboard protocol

[postsolar]

Fish version: 3.7.1-1795-g8639d7e45-dirty

Currently fish does not support full CSIu protocol, which leads to issues with correctly detecting keyboard input. This was discussed here and here.

The latter link also includes reproduction info.

[faho]

Fish does read shift-backspace in kitty, so this appears to be a foot issue to me?

I do not understand why shift-backspace would "resolve to text-only". I.e. even if you count backspace as "text", which it kinda is (ASCII 0x08), shift-backspace certainly is a modified key, so https://codeberg.org/dnkl/foot/issues/1642#issuecomment-1660461 kinda contradicts itself.

If foot thinks shift-backspace is easily mistyped and doesn't want to present it differently, that's their right.

Enabling the full shebang has issues in that it's a lot more to read and also breaks things much more when e.g. fish crashes without disabling the protocol - run kitten show_key -m kitty in bash and then kill -9 it, you'll end up with basically a broken shell. The current version will send some keys in a way bash won't understand, but the basics will still work so you can regain a functional shell via reset (or starting a new fish).