[Story Player] Optimize Story loading strategy

[gmajoulet]

Objective

Improve LCP of the visible Story when using the show() method on the non first Story.
Calling show() to display the n-th story should be just as fast as displaying the first Story.

How

Loading a Story is using quite some bandwidth, and loading several at once introduces significant delays on the LCP of the visible one.
We should always:

1. Load the visible story, and wait for storyContentLoaded event, then
2. Load the N + 1 story with a prerender visibilitystate. No need to prerender the previous one until user tries swiping to it

cc @ampproject/wg-stories

[gmajoulet]

Duplicate of #31212