PR Summary

The PR effects only Windows behavior.

• With UseNewEnvironment parameter Start-Process run a child process with environment variables without modifications in current session (state before the session start).
  See Start-Process -UseNewEnvironment provides an environment that is missing crucial standard environment variables while not supporting passing a new environment #4671 and PowerShell Committee conclusion UseNewEnvironment restores environment variables for child process #10745 (comment)
• Without UseNewEnvironment parameter Start-Process run a child process with current environment variables from current session. (This allow us easily to add -Environment parameter late)

PR Context

Replace #10561
Fix #3545
Fix partially #4671

PowerShell Committee conclusion #10745 (comment)

PR Checklist

• PR has a meaningful title
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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed: New UseNewEnvironment parameter behavior of Start-Process cmdlet on Windows MicrosoftDocs/PowerShell-Docs#5774
• Testing - New and feature
  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
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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 PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).