Add feature flag for turning off keyboard protocols

To work around terminal bugs.

The flag "keyboard-protocols" defaults to "on" and enables keyboard protocols,
but can be turned off by setting "no-keyboard-protocols".

This has downsides as a feature flag - if you use multiple terminals and
one of them can't do it you'll have to disable it in all,
but anything else would require us to hook this up to env-dispatch,
and ensure that we turn the protocols *off* when the flag is disabled.

Since this is a temporary inconvenience, this would be okay to ask
zellij and Jetbrains-with-WSL users.

tests/checks/status.fish

@@ -59,6 +59,7 @@ status features
 #CHECK: ampersand-nobg-in-token on  3.4 & only backgrounds if followed by a separator
 #CHECK: remove-percent-self     off 4.0 %self is no longer expanded (use $fish_pid)
 #CHECK: test-require-arg        off 4.0 builtin test requires an argument
+#CHECK: keyboard-protocols      on  4.0 Use keyboard protocols (kitty, xterm's modifyotherkeys
 status test-feature stderr-nocaret
 echo $status
 #CHECK: 0