Keymap: Custom (fabian) layout for Iris and Planck (#3460)

Keymap: Custom (fabian) layout for Iris and Planck  (#3460)

* fabian layout

* added MOUSECURSOR layer that gets activated by holding space

* cleanup, meh and tilde and grave as separate keys, toggle switch for mousecursor layer

* merged default

* #pragma once

* including QMK_KEYBOARD_H instead of various imports

* using layer toggle instead of a function

* deleted obsolete rules.mk (was part of the old makefile system)

* #pragma once

* including QMK_KEYBOARD_H instead of various imports

* use ifndef KEYBOARD_planck_rev6 instead of ifdef KEYBOARD_planck_rev5