style: file nesting

[Flash0ver]

Nest C# files that are all parts of a partial type.

Unfortunately, Rider does not support Nesting Files based on Wildcards: https://www.jetbrains.com/help/rider/File_Nesting_Dialog.html
So we need to be quite explicit.

After: #4834

See also: #4834 (comment)

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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• style: file nesting by Flash0ver in #4894

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.88%. Comparing base (5df4eed) to head (74bd2b8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4894      +/-   ##
==========================================
+ Coverage   73.87%   73.88%   +0.01%     
==========================================
  Files         494      494              
  Lines       17868    17868              
  Branches     3509     3509              
==========================================
+ Hits        13200    13202       +2     
+ Misses       3808     3807       -1     
+ Partials      860      859       -1     

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