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

keyboards/mxss/readme.md

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+# MxSS - Polycarb 65% Kit for MX/SMK
+
+![MxSS - Polycarb 65% Kit for MX/SMK](https://i.imgur.com/WDTWcmU.jpg)
+
+### Information:
+
+ - Case: Frosted Polycarbonate, CNC milled
+ - Plate: Brass, mirror-finished and electroplated
+ - Weight: Same as plate
+ - PCB: Custom designed for the MxSS by kawasaki161, White solder mask and ENIG finish
+
+### Details:
+
+ - 2.9 degrees angle on the case
+ - Top mount plate
+ - Center USB, Type-B Mini
+ - MX and SMK (White and Orange) switch support
+ - Holtite support
+ - RGB underglow
+ - 1.5kg with plate and weight
+
+Keyboard Maintainer: [MxBlue](https://github.com/mxblu)  
+
+Hardware Supported: Custom PCB, ATMega32u4
+
+Hardware Availability: https://geekhack.org/index.php?topic=94986.0
+
+Make example for this keyboard (after setting up your build environment):
+
+    make mxss:default
+
+See [build environment setup](https://docs.qmk.fm/build_environment_setup.html) then the [make instructions](https://docs.qmk.fm/make_instructions.html) for more information.
+
+# Front LED Operation
+
+The MxSS PCB has 2 front RGB LEDs which has a separate brightness and mode of operation to the rest of the RGB LEDs. These are controlled through the supplied custom keycodes.
+
+There are 3 modes of operation:
+
+ - FLED_OFF - Front LEDs stay off
+ - FLED_RGB - Front LEDs are part of the standard RGB LED modes, only works correctly with rainbow modes (will fix on request)
+ - FLED_INDI - Top front LED represents Caps Lock status, bottom LED represents current layer
+ 
+Colors for FLED_INDI mode are hardcoded as hue/saturation values, the caps lock color can be found in mxss_frontled.h, the layer colors are defined in keymap.c (see default/keymap.c for example).
+
+## Custom Keycodes
+
+ - FLED_MOD - Cycle between the 3 modes (FLED_OFF -> FLED_RGB -> FLED_INDI)
+ - FLED_VAI - Increase front LED brightness
+ - FLED_VAD - Decrease front LED brightness
+ 
+ # Further Notes
+ 
+ As SAFE_RANGE is used for defining the custom keycodes seen above, please use NEW_SAFE_RANGE as the starting value for any custom keycodes in keymap.c, as per the example.