Sort system usings first

[martincostello]

Sort system usings first, rather than strict alphabetical.

See https://github.com/domaindrivendev/Swashbuckle.AspNetCore/pull/2768#issuecomment-2053936433.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.66%. Comparing base (9e77996) to head (65fa949).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2790   +/-   ##
=======================================
  Coverage   91.66%   91.66%           
=======================================
  Files          91       91           
  Lines        3010     3010           
  Branches      517      517           
=======================================
  Hits         2759     2759           
  Misses        251      251           

Flag	Coverage Δ
Linux	91.66% <ø> (ø)
Windows	91.66% <ø> (ø)
macOS	91.66% <ø> (ø)

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