Never emit the GenerateShapeAttribute<T> into a user's assembly

[AArnott]

It isn't necessary, since it only triggers source generation within the compilation of the code that applies it. It's also problematic because some runtimes (e.g. .NET Framework and unity) do not support generic attributes and reflection APIs throw when they encounter it.

[eiriktsarpalis]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.61%. Comparing base (7769d18) to head (e9860e0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #111   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files         191      191           
  Lines       12138    12138           
  Branches     1585     1585           
=======================================
  Hits        11484    11484           
  Misses        652      652           
  Partials        2        2           

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