EntityLookupSystem extra helper methods

[gradientvera]

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Would be nice if EntityLookupSystem could have some more convenience methods

• Predicate methods for most queries (with optional generic state argument)
• Method for getting all entities in a grid's specific tile
  • Now, I realize that MapGrid has some methods to at least get anchored entities in tiles, but honestly I really dislike that because it's all over the place!
  • Ideally, we should be able to just depend on EntityLookupSystem alone for all our entity lookup purposes. We should centralize everything there!
    • Actually, maybe content can implement extension methods for EntityLookupSystem, for their own purposes... We have the technology.....

I can look into coding some of these myself, if needed