User Command (Script) Menu

The menu (commands / scripts) is configurable in `Configuration_adv.h`.

- Added conditionals.
- Changed script handling.
- Slimmed and working!
- Added Status message.
- Returning to lcd status screen now!
- Example Configs Updated.
- Cleanups by @thinkyhead

Marlin/language_en.h

@@ -156,6 +156,9 @@
 #ifndef MSG_LEVEL_BED
   #define MSG_LEVEL_BED                       _UxGT("Level bed")
 #endif
+#ifndef MSG_USER_MENU
+  #define MSG_USER_MENU                       _UxGT("Custom Commands")
+#endif
 
 #if ENABLED(AUTO_BED_LEVELING_UBL)
   #ifndef MSG_UBL_UNHOMED