Add MxSS keyboard (#3335)

Add MxSS keyboard (#3335)

* Added basic MxSS support

* Fixed split RSHFT for ISO layouts

* Updated readme.md for MxSS

* Added initial support for individual control of front RGB LEDs

* Changed RGBLED color selection to work using hue and saturation rather than RGB
Added code for LED state change on layer change

* Avoid needing an entire 8 bits to store the brightness value

* Added custom keycodes, along with their handlers

* Added EEPROM storage for front LED config

* Fixed up ability to use QMK Configurator and updated readme.md

* Applied suggested changes from pull request: https://github.com/standard/standard/issues/452

Updated name in license descriptions
Updated layouts to snake case
Corrected mistakes in info.json
Updated layer_colors to a weak attributed array in mxss.c

* Defined a new safe range for custom keycodes in keymap.c