Basis is missing the IDENTITY and ortho constants

[aaronfranke]

Godot version: 3.1 beta 10

OS/device including version: All

Issue description:

While Vector3 has constants such as ONE and UP, Basis is missing the IDENTITY constant and the otho constants. These constants are already present in C# here, but not in GDScript. Related PR: #14704

I will fix this later if nobody else does, but it will have to wait for 3.2. It's very non-critical.

[neikeq]

Isn't Basis() the same as Identity?

[aaronfranke]

@neikeq Yes, you're right, it is! Though, there should still be an IDENTITY property to avoid confusion, just as Vector3 has ZERO. Also, I noticed that in C#, there is no parameterless Basis constructor.

[neikeq]

C# doesn't allow value types to have parameterless constructors (I think CLR does allow it though).