Add `Processor#key_symbol`

It receives an array returned by `Terminal#get_wch`.
It's for `return unless key[1] < 127`.
https://github.com/khwon/loco/blob/9f55fb7db3b67dfed3a99278416f25d574d1ef0a/termapp/processors/select_board_menu.rb#L83
It seems `matched?` won't affected, but I think check is needed. Otherwise we can check `char` on `matched?`.