add supertypes(T) function

[stevengj]

Closes #34362, adding this function to InteractiveUtils along with subtypes.

Unlike subtypes, returns a tuple instead of an array, just in case we ever decide that we want this function to work statically (since it can).

[JeffBezanson]

One reason to discourage code from depending on supertypes is that they are not part of API stability guarantees --- we are allowed to insert new abstract types in the hierarchy in 1.x. So enabling compile-time tricks with them might not be for the best.

[stevengj]

@JeffBezanson, I'm not sure I understand your point. You can't insert new abstract types into the type hierarchy of a given binary after compilation, so compile-time usage of supertypes are perfectly safe, no? Besides, the supertype function itself is already @_pure_meta, so the horse is out of the barn.

(Hard-coding dependence on the specific supertypes might be bad, but this doesn't apply to code that uses a function like supertype.)

[JeffBezanson]

It's safe, just subject to breakage. But ¯\_(ツ)_/¯