Fix duration updates when media duration changes

[wanxiankai]

Summary

• detect duration changes during progress/timeupdate events and dispatch a durationchange event when providers update duration without emitting one
• keep the duration cache reset on loadstart so playlist/internal media changes can refresh metadata
• make the server-safe test globals compatible with read-only global accessors in newer Node versions

Fixes #1987.

Tests

• npm run lint
• npm test
• npm run types