fix: type properly tracks_ on TrackList

[nolway]

Description

Use a generic in TrackList to provide better typing for _tracks.

Specific Changes proposed

The TrackList type has changed, I added a generic to provide stronger typing for the kind of track it stores.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

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[FR073N]

I came across the same problem, this fix seems to be the solution.