spacebar shortcut to pause a Web Story

### Description

A user of the Web Stories for WordPress plugin has requested for a keyboard shortcut for pausing a Web Story. This would be using the spacebar.

The user thinks that by adding this functionality the UX for the web user would be much better. Currently, the user has to click in the top right corner on the pause icon. Since the user can use the arrows on his keyboard to go to the next ‘page’ in the web story, it would be easier for them to be able to pause the web story too by pressing the spacebar.
Users are used to pressing the spacebar to pause videos on YouTube, Instagram, Facebook, Netflix…

### Alternatives Considered

N/A

### Additional Context

Related topic in the Web Stories WordPress support forum:
https://wordpress.org/support/topic/2-feature-requests-press-spacebar-to-pause-a-web-story-restart-web-story/

Start by tracing the existing Web Stories keyboard handling for arrow navigation and the top-right pause control. Add the spacebar behavior so users can pause the story from the keyboard, while preserving the existing navigation behavior, and verify it in the Web Stories experience.

Written by the indexing model from the issue text.