Extended & human-friendly keys

See the changelog additions for user-visible changes.

Since we enable/disable terminal protocols whenever we pass terminal ownership,
tests can no longer run in parallel on the same terminal.

For the same reason, readline shortcuts in the gdb REPL will not work anymore.
As a remedy, use gdbserver, or lobby for CSI u support in libreadline.

Add sleep to some tests, otherwise they fall (both in CI and locally).

There are two weird failures on FreeBSD remaining, disable them for now
https://github.com/fish-shell/fish-shell/pull/10359/checks?check_run_id=23330096362

Design and implementation borrows heavily from Kakoune.

In future, we should try to implement more of the kitty progressive
enhancements.

Closes #10359

tests/checks/variable-assignment.fish

@@ -1,4 +1,4 @@
-# RUN: %fish %s
+# RUN: %fish %s | %filter-ctrlseqs
 
 # erase all lowercase variables to make sure they don't break our tests
 for varname in (set -xn | string match -r '^[a-z].*')