Break out support for Formbot T-Rex3 from previous versions

Marlin/src/core/boards.h

 #define BOARD_AZTEEG_X3_PRO     68    // Azteeg X3 Pro
 #define BOARD_ULTIMAIN_2        72    // Ultimainboard 2.x (Uses TEMP_SENSOR 20)
 #define BOARD_RUMBA             80    // Rumba
-#define BOARD_FORMBOT           81    // Formbot version 1. (A Rumba derivative)
+#define BOARD_FORMBOT_TREX2     81    // Formbot version 1
+#define BOARD_FORMBOT_TREX3     82    // Formbot T-Rex 3 revision
 #define BOARD_BQ_ZUM_MEGA_3D    503   // bq ZUM Mega 3D
 #define BOARD_MAKEBOARD_MINI    431   // MakeBoard Mini v2.1.2 is a control board sold by MicroMake
 #define BOARD_TRIGORILLA_13     343   // TriGorilla Anycubic version 1.3 based on RAMPS EFB