Reorganize TestResuls-code into format-specific files

[fflaten]

PR Summary

• Splits code specific to a single format into format-named files (Add support for NUnit3 XML-reports #2208 (comment))
  • Shared helpers and public functions are left in TestResults.ps1 as they're used for both NUnit 2.5 and 3. Same might apply to JUnit4 and ex. JUnit5 in the future.
• Moves schema-files to format-named folder in dist (Add support for NUnit3 XML-reports #2208 (comment))
• Updates release.ps1 to verify all mandatory files are available in build
• Moves Get-HumanTime to Output.ps1 where it's used
• Deletes unused Exit-WithCode helper.

PR Checklist

• PR has meaningful title
• Summary describes changes
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• Tests are added/update (if required)
• Documentation is updated/added (if required)

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[fflaten]

Something to consider: Deprecate public Export/ConvertTo-*UnitResult functions for NUnit and JUnit and expose Export-PesterResult with a new -AsString/PassThru parameter.