Add classes and fields to ForgeHooks, which might be used by hacky mods

[rikka0w0]

This fix makes https://github.com/MinecraftModDevelopmentMods/AdditionalLootTables/tree/master-1.15.2 work.
By design, AdditionalLootTables 1.15.2 should also work in 1.14.4. Without coremoding, in order to append items to an existing loottable, Forge mods have to use reflection on ForgeHooks.lootContext.

[kitlith]

Don't have to deal with this right now, but I wonder how #139 would change things, since that isn't using the LootTableContext struct anymore internally. EDIT: it would work, but kinda only coincidentally since it's setting the name, which is the only thing the relevant hook looks for in that PR. :/

Overall, the changes here make sense to me.