feat(captions): add auto-scroll support for active captions

[Lokaprathap]

Link to related issue (if applicable)

Summary of proposed changes

Summary

This PR introduces an optional captions auto-scroll plugin that keeps the active caption line in view during playback.

Motivation

For long captions or accessibility use-cases, users currently need to manually scroll captions to follow along. Auto-scrolling improves readability and usability without altering existing captions behavior.

Implementation

• Added a lightweight captions-autoscroll plugin
• Plugin initializes only when captions are enabled
• Does not affect default captions rendering or controls
• Fully opt-in and non-breaking
• Integrated via plyr.js constructor without affecting existing flow

Testing

• Verified with HTML5 video captions
• Confirmed captions remain unchanged when plugin is disabled
• No regressions observed in existing captions behavior

[Lokaprathap]

Hi maintainers,
Thanks for reviewing this PR.
Happy to make any changes or improvements if needed.