feat: export the base class MathArray、Matrix、Vector

[GuoBinyong]

I'm a library developer, and I often need to inherit MathArray, Matrix, and Vector to implement some subclasses, and I often use these base classes as type constraints, but @math.gl/core doesn't expose these classes

[ibgreen]

I'm OK with exposing these as experimental / underscore exports.

They will very likely remain stable but prefer not to commit to it at this point.

Is that OK? It should unblock you.