[Refactor] rewrite the spec text so that .finally observably calls .then

[ljharb]

This is a PR demonstrating the spec refactor that makes .finally, like .catch, observably call into .then.

Pros:

• consistency with .catch
• make subclassing easier in that you still only have to override .then (which might be useful in particular because the callback logic in finally is complex)
• Less spec text

Cons:

• adds an observable [[Get]] and [[Call]] of "then"
• makes two new spec-created functions available to user code (which normally is only done with the new Promise executor's resolve and reject function), thus preventing them from being optimized out in implementations.

[dead-claudia]

@ljharb

• makes two new spec-created functions available to user code (which normally is only done with the new Promise executor's resolve and reject function), thus preventing them from being optimized out in implementations.

Not if both then and finally are on an unmodified Promise.prototype, neither shadowed. At least V8 does this in TurboFan (from looking at the source) for then in particular, so I feel it is possible.

[ljharb]

@isiahmeadows yes, this is an optimization that an implementation can certainly perform in the best case, but observability needs to be judged for the worst case, not the best case.