Add static methods to C# classes

[WhistleWiz]

Currently, most classes in C# only have instance methods available to them.

This goes against most C# built-in types (e.g. System namespace), which use static methods for many operations (Godot's Vector3 uses instance method a.Dot(b); while System.Numerics uses static method Vector3.Dot(a, b);).

From my understanding, this is so the GDScript matches 1:1 with C# as closely as possible. My proposal is to be able to use both versions. Static methods make for much cleaner code, and would follow built-in types more closely for C# developers. Certain methods use confusing parameter order too, like CubicInterpolate which almost feels like it follows a random order.

Implementing this change wouldn't break anything that already exists, but would be a tedious endeavor. There are 3 ways of doing it:

1. Copying the code with slight changes from the instance methods
2. Have the static method call the corresponding instance method internally
3. Have the instance method call the static one internally

The first would create a lot of duplicate code which doesn't help with future maintenance, while 2 and 3 are basically wrappers which would require minimal change.

Unfortunately I am new user of Godot so I'm not very familiar with the GDScript side of things, so I do not know if there are more challenges to these implementations. I believe #4164 adresses similar issues, but not exactly the same.