Function check_for_mutating_key_press

fn check_for_mutating_key_press(
    os: OperatingSystem,
    cursor_range: &CCursorRange,
    text: &mut dyn TextBuffer,
    galley: &Galley,
    modifiers: &Modifiers,
    key: Key,
) -> Option<CCursorRange>

Returns Some(new_cursor) if we did mutate text.