Delete with a modifier produced no input at all. Since the alt handling
was limited to single characters, alt-delete stopped sending the bogus
`ESC delete` and fell through to nothing, because delete was missing from
the modified special keys table next to insert, pageup and pagedown.
Modified delete now goes out as `CSI 3 ; <modifier> ~`, the same
encoding xterm and alacritty use.
Closes#64721