First PR for KBD6x HHKB layout keymap (#4704)

First PR for KBD6x HHKB layout keymap (#4704)

* initial commit for Othi's HHKB layout keymap,
covering multiple modifiers hold and vim-like support and german characters. If there's
any suggestion i can be reached via mnpqraven on github or Othi#6661 on
discord

* added readme.md. TODO: update readme.md

* changed to default quantum keycode for `KC_TRNS`,
removed `PREVENT_STUCK_MODIFIERS`,
fixed tap dance using one shots
TODO: unicode implementation at https://docs.qmk.fm/#/feature_unicode

* keymap documentaion(readme) added

* unicode init
TODO: figure out what made the compose sequence not running

* update unicode and readme