untagged-c11a684819fb355e28ff: Support more options for predefined items.

By setting the "type" option to "external::spigot", you can use Spigot's deserialization of items for predefined items. The data that is passed to the deserializer is the contents of the subsection called "data". Note, the logic behind this deserialization is not controlled by MagicSpells, so no, I cannot tweak how spigot handles deserialization, but I will later support further deserializers.

Introduce new option to SpawnTntSpell
The option is called cancel-gravity which defaults to false. When set to true, and your version of minecraft supports it, the tnt spawned will not have gravity