Added the ability for mods to use the Difficulty class

[hydrophobis]

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It works but it is slightly janky. The modder must call

public void mindustry.content.Difficulties.addDifficulties(Difficulty...difficulties)

after their own .load() to add each of their difficulties
I would have integrated it in the ModClassLoader but i wasnt sure where to put it or if it even would work in that file so I didn't do this