Add a GHA for unexported, unused objects

[MichaelChirico]

Eventually, we can remove this GHA once #1513 is fixed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (3539ef4) to head (6a58a58).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2816      +/-   ##
==========================================
- Coverage   99.64%   99.64%   -0.01%     
==========================================
  Files         127      127              
  Lines        7109     7090      -19     
==========================================
- Hits         7084     7065      -19     
  Misses         25       25              

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[IndrajeetPatil]

Really cool!

Love that this already helped detect some unused things :)