Add isCallable() and isConstructable()

[jcbhmr]

https://github.com/caitp/TC39-Proposals/blob/trunk/tc39-reflect-isconstructor-iscallable.md

☝ Basically that.

Use case:

if (isConstructable(thing)) {
  return new thing()
} else if (isCallable(thing)) {
  return thing()
} else {
  return thing
}

[mesqueeb]

Seems good to me! PR welcome