fromfilename: Support 'track' prefix when parsing track number

[snejus]

fromfilename: Support "track" prefix in track number parsing

The fromfilename plugin's filename-matching regex is extended to recognise filenames like track01.m4a or track 2.m4a, where a literal "track" prefix precedes the track number.

[Copilot]

Pull request overview

PR make fromfilename plugin understand filename start with literal track before track number (like track01.m4a / track 2.m4a).

Changes:

• Extend fromfilename filename regex to accept optional track prefix before digits.
• Add pytest coverage for track01 and track 2 filename cases.
• Add changelog entry for new parsing behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
beetsplug/fromfilename.py	Update track-only regex to allow optional track prefix before number.
test/plugins/test_fromfilename.py	Add tests for track01.m4a and track 2.m4a parsing.
docs/changelog.rst	Document new track-prefix support in Unreleased features.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.85%. Comparing base (44ffbdf) to head (2698044).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6557   +/-   ##
=======================================
  Coverage   71.85%   71.85%           
=======================================
  Files         156      156           
  Lines       20181    20181           
  Branches     3211     3211           
=======================================
  Hits        14501    14501           
  Misses       5001     5001           
  Partials      679      679           

Files with missing lines	Coverage Δ
beetsplug/fromfilename.py	92.64% <ø> (ø)

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[aereaux]

This is a pretty small change but it looks good and would be useful for me!