Implement Shields Down's form change; Fix wild Rotom forms not appearing

Shields Down is only partially implemented; the form changing aspect is implemented, but the immunity to statuses is not yet.
Minior's alternate colored forms can now be encountered in the wild.
Rotom's appliance forms should *finally* appear in the wild correctly now.
And a minor fix for Zen Mode activating incorrectly.

src/field/arena.ts

@@ -166,6 +166,20 @@ export class Arena {
             return 2;
         }
         break;
+      case Species.ROTOM:
+        switch (this.biomeType) {
+          case Biome.VOLCANO:
+            return 1;
+          case Biome.SEA:
+            return 2;
+          case Biome.ICE_CAVE:
+            return 3;
+          case Biome.MOUNTAIN:
+            return 4;
+          case Biome.TALL_GRASS:
+            return 5;
+        }
+        break;
       case Species.SCATTERBUG:
       case Species.SPEWPA:
       case Species.VIVILLON: