Add afterLoadContent() to Mod.java that called between Mod.loadContent() and Mod.init()

[zeutd]

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it makes a mod can create content based on other unspecific mod's content

• [x] I have read the contribution guidelines.
• [x] I have ensured that my code compiles, if applicable.
• [x] I have ensured that any new features in this PR function correctly in-game, if applicable.