Method and field names for singletons should be unified

[haykam821]

Classes using a singleton pattern generally store an instance of the class in a private field and provide a public method for accessing the instance. However, the names of these methods and fields are not consistent, so these names should be changed.

• Find all classes with a singleton pattern
• Determine which method and field names to use
• Change the method and field names

For example, BiomePlacementModifier and SquarePlacementModifier both use an INSTANCE field and of() method.

[liach]

of() is an alternative to a no-arg constructor for immutable classes, like List.of(). Otherwise, I would recommend getInstance() if the class is mutable, such as the fabric loader or Minecraft client instance. Thus, immutable no-arg instance should be called EMPTY or DEFAULT, while the getInstance() ones should be called INSTANCE (mutable, but it's static final field)

[apple502j]

I believe, at least here, consistency within a package is more important than consistency within Yarn as whole. I do think getInstance should be used for things that must be singleton, and of should be used where it is just performance optimization (and similar classes in the package, taking arguments, have the of method.) I think other placement modifiers use of, in that case all modifiers should use of.