QOL: Add command autocompletion when typing keywords preceded by "/" in the chat.

Works both for in-game chat and lobby chat.

Typing just "/" will preview all commands like "/help" does.

If multiple options are present, you're able to select between them with up and down arrow keys.

Pressing enter once autocompletes with the selected command. You'll be required to press enter again to send it.

Ideally it should be able to work with custom commands added by scripts.

Can be disabled in settings.