Match default export of plugin and `plugin` object of flat configs

[soren121]

Fixes #3212

The plugin's default export is not equal by reference to the plugin object defined on flat configs. This can cause issues for ESLint's plugin normalization in some scenarios. This PR changes the default export to equal the plugin object.

I've added tests for this change, and I also added missing tests for the flat config exports.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.77%. Comparing base (01c9eb0) to head (12d770c).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3213       +/-   ##
===========================================
+ Coverage   82.25%   95.77%   +13.51%     
===========================================
  Files          94       83       -11     
  Lines        4283     3691      -592     
  Branches     1478     1332      -146     
===========================================
+ Hits         3523     3535       +12     
+ Misses        760      156      -604     

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[jankanty]

Is there a way to progress with this PR? It blocks usage of default configs with flat structure.

[Nivl]

cc @ljharb