0x08 (BS) was treated as plain backspace alongside 0x7f (DEL). On
Windows Terminal, Backspace sends 0x7f and Ctrl+Backspace sends 0x08,
making ctrl+backspace bindings unreachable.
Now 0x08 matches ctrl+backspace (and ctrl+h, same byte) but not plain
backspace. 0x7f remains plain backspace.
fixes#2139
Ctrl+\ sends ASCII 28 (File Separator) in legacy terminals. This is
commonly used as SIGQUIT in Unix.
Ctrl+] sends ASCII 29 (Group Separator) in legacy terminals. This is
commonly used as the telnet escape character.
Ctrl+_ sends ASCII 31 (Unit Separator) in legacy terminals. On US
keyboards, - and _ are on the same physical key, so this also functions
as an alias for Ctrl+-.
In legacy terminal mode (non-Kitty protocol), Alt+key is sent as ESC
followed by the key character. This was only supported for specific keys
(space, backspace, arrows) but not for regular letters.
Add support for Alt+letter sequences to enable keybindings like Alt+Y.
