[amp-story-player] Introduce render() method ♻️

[Enriqe]

Introduces render() method which consolidates a lot of the logic from different methods, effectively reducing the amount codepaths.

Follow up for #32115
Closes #31212
Closes #31010
Closes #31445
Closes #31562
Closes #31925
Closes #31738

[gmajoulet]

This is fantastic ❤️

We discussed the review offline but for transparency:

1. remove addingStories and look at if the current StoryDef has a connected iframe instead. That makes it a bit more automatic and easy to use it's cool.
2. Remove the limit, I think with the early return in the loop we don't really need this kind of optimization, especially if it makes the code a bit easier
3. Move the early return a bit higher so we don't even create a render promise