move setup scripts to be nested inside scripts

[sunshowers]

Much of this has been adapted from #2018, with attention paid towards short-term back compat ("scripts.setup" instead of "script.setup").

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 42 lines in your changes missing coverage. Please review.

Project coverage is 78.30%. Comparing base (b1409cd) to head (e24f49d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/config/config_impl.rs	74.12%	37 Missing ⚠️
nextest-runner/src/config/scripts.rs	76.19%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2379      +/-   ##
==========================================
- Coverage   78.32%   78.30%   -0.03%     
==========================================
  Files         106      106              
  Lines       22536    22660     +124     
==========================================
+ Hits        17652    17743      +91     
- Misses       4884     4917      +33     

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[trunk-io]

     

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