Add Zygarde to FormInfo.BattleMegas (#4667)

`IsBattleMegaForm` already checks for Mega Zygarde, but since it's only
called if the species is in `BattleMegas`, both `IsBattleOnlyForm` and
`IsMegaForm` currently return false for Mega Zygarde

PKHeX.Core/Legality/Tables/FormInfo.cs

@@ -282,6 +282,7 @@ public static bool IsFormChangeable(ushort species, byte oldForm, byte newForm,
         (int)Barbaracle,
         (int)Dragalge,
         (int)Hawlucha,
+        (int)Zygarde,
         (int)Drampa,
         (int)Falinks,