Add feature flag for turning off keyboard protocols

[faho]

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.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[zanchey]

My only concern is that if the broken terminals get fixed, how do users know to turn the flag back off?

[faho]

I guess they'll have to try regularly?

That's fundamental to anything that allows you to turn it off - even if this were a shell variable you'd have to remove it again if your terminal is fixed.

Tho at a certain point we'll phase this flag out again (as all feature flags should be), which would re-enable it.

And it's not like not having kitty keyboard is some horrible fate - we've not had it for decades, you can live without it for a bit longer. Sure, it's nicer to have it, but on the whole if the alternative is "my terminal is unusable" like with zellij? The choice is clear.

[zanchey]

OK, I think it's fine to include this as an escape hatch for 4.0.

[krobelus]

For 4.0 sure but as of today it's not needed in master.
It's a shame we can't use fish | sed ... to filter out problematic sequences.