Fix return type

[bakkot]

The current spec misuses ~return~ completions. Those are use for propagating a literal syntactic return in an ES function up through any intermediate syntax (which is handled by the ? macro) to the [[Call]] AO which invoked the function, where it is unwrapped into a normal completion (in step 10.2.1 of that algorithm).

Built-in functions do not need to deal with that propagation, so they can (and do) just return a normal completion holding the value in question directly. The [[Call]] semantics for built-in functions have no corresponding return->normal unwrapping.

(The current spec text ends up propagating a return completion to the caller - that is, as currently written, it would cause a return from the code which called fromAsync.)