Terminals that rewrite Cmd+Backspace to Ctrl+U already reach
unix-line-discard. Kitty keyboard protocol and xterm modifyOtherKeys
terminals instead report Cmd as the super modifier bit, producing CSI
sequences prompt_toolkit has no entry for — the raw bytes fall through
the VT100 parser and land in the buffer as literal text.
Alias those to the readline kill bindings prompt_toolkit already ships.
Backspace is a CSI-u codepoint (127); ForwardDelete is a CSI tilde key,
so its modifier rides in the CSI 3 ; mod ~ form rather than CSI-u.
Ctrl+ForwardDelete keeps its own binding — that is delete-word on
Linux/Windows, not kill-line.
