Parse CSI codepoint;modifiers u sequences from terminals that support
the Kitty keyboard protocol (Kitty, Ghostty, WezTerm, iTerm2, etc).
Maps special key codepoints (Tab, Enter, Escape, Backspace, F1-F12,
arrows, etc.) to existing KEY_* constants, and decodes modifier
bitfields to MOD_* flags.

Protocol overview: https://sw.kovidgoyal.net/kitty/keyboard-protocol/
Key encoding: https://sw.kovidgoyal.net/kitty/keyboard-protocol/#key-event-encoding
Functional keys: https://sw.kovidgoyal.net/kitty/keyboard-protocol/#functional-key-definitions
Modifier encoding: https://sw.kovidgoyal.net/kitty/keyboard-protocol/#modifiers