Add an event on autoadvance delay change.

[queenvictoria]

Description

This PR adds a new event, autoadvance-delaychange, to the videojs-playlist plugin. The event is triggered whenever the auto-advance delay value is changed (including when it is set to null to disable auto-advance). This allows external code to listen for and respond to changes in the auto-advance delay setting.

Specific Changes proposed

• Emit a custom autoadvance-delaychange event on the player whenever autoadvance_.delay is set or changed in src/auto-advance.js.
• Update the documentation (docs/api.md) to describe the new event and provide an example of how to listen for it.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
• Reviewed by Two Core Contributors