Core: Implement Nullable<T> template

[Repiteo]

Preliminary work for handling value-types as nullable in a manner that can be recognized more explicitly than Variant. The bindable versions of the aabb/plane functions had their return value changed from Variant to Nullable<Vector3> to demonstrate this capability without breaking compatability (bindings see Nullable as Variant atm).

Doesn't attempt to overhaul current bindings nor the type system; this is strictly for introducing this concept at its most basic level. The only exception is letting Nullable get bound in the same way as Variant, albeit specialized to its return type; that is, any bound Variant return type that represents specifically T and null can be 1-1 replaced with Nullable<T>.

[AThousandShips]

See also:

• Add RequiredPtr<T> to mark Object * arguments and return values as required #86079