PR Summary

This PR is the implementation for the ForEach-Object -Parallel feature based on RFC: PowerShell/PowerShell-RFC#194

PR Context

This new parameter set runs the ForEach-Object script blocks in parallel, for each piped input, on separate threads subject to a ThrottleLimit restriction. It can run synchronously where the cmdlet call does not return until all piped input has been processed. Or it can run asynchronously and return a job object immediately.

This is an experimental feature and must be explicitly enabled with the 'PSForEachObjectParallel' name.

PR Checklist

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• Breaking changes
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  • Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed: Add documentation for new ForEach-Object -Parallel parameter set MicrosoftDocs/PowerShell-Docs#4590
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  • N/A or can only be tested interactively
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    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
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    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).