[Keyboard] Monstargear XO87 Solderable support (#11716)

* Support for XO87 solderable version

* cleanup

* Remove abandoned code

* replaced KEYMAP with LAYOUT and moved LAYOUT macro to solderable.h.  deleted unneeded files.

* Update keyboards/monstargear/xo87/solderable/keymaps/via/keymap.c

* update info.json with missing keys

* Apply suggestions from code review

* Apply suggestions from code review

correct layout macro

keyboards/monstargear/xo87/solderable/rules.mk

@@ -0,0 +1,23 @@
+# MCU name
+MCU = at90usb646
+
+# Bootloader selection
+BOOTLOADER = lufa-dfu
+
+# Build Options
+#  change yes to no to disable
+#
+BOOTMAGIC_ENABLE = lite     # Virtual DIP switch configuration
+MOUSEKEY_ENABLE = yes       # Mouse keys
+EXTRAKEY_ENABLE = yes       # Audio control and System control
+CONSOLE_ENABLE = no         # Console for debug
+COMMAND_ENABLE = yes        # Commands for debug and configuration
+# Do not enable SLEEP_LED_ENABLE. it uses the same timer as BACKLIGHT_ENABLE
+SLEEP_LED_ENABLE = no       # Breathing sleep LED during USB suspend
+# if this doesn't work, see here: https://github.com/tmk/tmk_keyboard/wiki/FAQ#nkro-doesnt-work
+NKRO_ENABLE = yes           # USB Nkey Rollover
+BACKLIGHT_ENABLE = yes      # Enable keyboard backlight functionality
+RGBLIGHT_ENABLE = yes       # Enable keyboard RGB underglow
+BLUETOOTH_ENABLE = no       # Enable Bluetooth
+AUDIO_ENABLE = no           # Audio output
+BACKLIGHT_DRIVER = custom