Add Capability BUILD_PERCENT

Marlin/Marlin_main.cpp

       SERIAL_PROTOCOLLNPGM("Cap:LEVELING_DATA:0");
     #endif
 
+    // BUILD_PERCENT (M73)
+    #if ENABLED(LCD_SET_PROGRESS_MANUALLY)
+      SERIAL_PROTOCOLLNPGM("Cap:BUILD_PERCENT:1");
+    #else
+      SERIAL_PROTOCOLLNPGM("Cap:BUILD_PERCENT:0");
+    #endif
+
     // SOFTWARE_POWER (M80, M81)
     #if HAS_POWER_SWITCH
       SERIAL_PROTOCOLLNPGM("Cap:SOFTWARE_POWER:1");