fix: convert Tracklist length to a getter and fix event docs

[christriants]

Description

This PR fixes issue #9059 by ensuring the length property is properly exposed in the generated TypeScript definition files for TrackList.

Additionally, this PR fixes a typo, and corrects the @event tag for the labelchange event handler.

Specific Changes proposed

We can convert length to a getter method. By replacing the Object.defineProperty call in the constructor with get length() getter, we can make the property visible to the type generator.

After this change, the generated track-list.d.ts will include get length(): number;.

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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.31%. Comparing base (ec33990) to head (a012ba9).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9142      +/-   ##
==========================================
- Coverage   84.31%   84.31%   -0.01%     
==========================================
  Files         120      120              
  Lines        8154     8153       -1     
  Branches     1964     1964              
==========================================
- Hits         6875     6874       -1     
  Misses       1279     1279              

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[amtins]

LGTM

[welcome]

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