[Keymap] New kbd67/hotswap keymap for writing both code and math (#4933)

[Keymap] New kbd67/hotswap keymap for writing both code and math (#4933)

* Custom keymap.

* Fix magic layer, enable Unicode.

* Update README.

* Make Unicode config change keymap-local.

* Move magic to AltGr, simplifying layout.

* Changes per Drashna.

* Fix magic+esc = backtick!

* Change reset implementation per Drashna's suggestion.

* Add a few more symbols so long as the commit is open - √ℜℂℤ⊙ℚℕ and the like.

* Typo fix.

* Slight simplification, drop an extra variable.