Reuse original decorator only if not `Type(result) is Object` for methods, etc.

[dead-claudia]

The current specification relies on truthiness to determine whether to keep the decorator, but I think it might be a little cleaner to spec out checking the result of calling the decorator, instead of relying on truthiness. TypeScript already follows this logic itself, where it requires TypedPropertyDescriptor<T> | void for the return type. This change would also put it more in line with the rest of the spec.

A few minimal examples to demonstrate:

(Note: %variable refers to an internal variable here.)

• Check typeof result === "function" for class decorators

  // Original
  @decorator
  class C {}
  
  // Current
  class C {}
  C = decorator(C) || C
  
  // Proposed
  class C {}
  let %temp = decorator(C)
  if (typeof %temp === "function") C = %temp

• Check typeof result === "object" for method and accessor decorators.

  class C {
    @decorator
    method() {}
  }
  
  // Current
  class C {
    method() {}
  }
  let %desc = Object.getOwnPropertyDescriptor(C.prototype, "method")
  %desc = decorator(C.prototype, "method", %desc) || %desc
  Object.defineProperty(C.prototype, "method", %desc)
  
  // Proposed
  class C {
    method() {}
  }
  let %desc = Object.getOwnPropertyDescriptor(C.prototype, "method")
  let %result = decorator(C.prototype, "method", %desc)
  %desc = typeof %result === "object" ? %result : %desc
  Object.defineProperty(C.prototype, "method", %desc)