More data in UBL class, make it a static class

- Make all `unified_bed_leveling` data/methods static
- Move some UBL-related variables into the class
- Replace `map_[xy]_index_to_bed_location` with `mesh_index_to_[xy]pos`

Marlin/Marlin_main.cpp

@@ -3221,7 +3221,7 @@ inline void gcode_G4() {
    */
   inline void gcode_G12() {
     // Don't allow nozzle cleaning without homing first
-    if (axis_unhomed_error(true, true, true)) { return; }
+    if (axis_unhomed_error(true, true, true)) return;
 
     const uint8_t pattern = code_seen('P') ? code_value_ushort() : 0,
                   strokes = code_seen('S') ? code_value_ushort() : NOZZLE_CLEAN_STROKES,