Add py typed to beetsplug

[snejus]

PR Summary

This change adds beetsplug/py.typed to mark beetsplug as a typed Python package under PEP 561.

High-level impact

• Tells type checkers that beetsplug ships type information.
• Improves typing support for code that imports beetsplug (for example, beetcamp).
• Has no runtime or architecture changes beyond package metadata.

[Copilot]

Copilot wasn't able to review any files in this pull request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.54%. Comparing base (0600d4e) to head (3e7d1d4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6670   +/-   ##
=======================================
  Coverage   72.54%   72.54%           
=======================================
  Files         162      162           
  Lines       20787    20787           
  Branches     3288     3288           
=======================================
  Hits        15079    15079           
  Misses       4983     4983           
  Partials      725      725           

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