Warn about common namespace collisions

[JavidPack]

Any mod with namespaces named Projectile/NPC/Item/etc will pretty much always cause issues. We should warn against that

[steviegt6]

+1
I agree, warning against namespaces that would mess with accessing vanilla types should be warned against (at least important ones like Main, NPC, Item, Projectile, Dust, etc.).
Should probably incorporate Main and other ones just to be safe, because you never know what people will give call their namespaces sometimes...