PR Summary

Fix #15466

Continue #15260

EnsureExtendedPrefix is copy-pasted from .Net Runtime repository.

PR Context

PR Checklist

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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
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  • Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
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  • Documentation needed
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• Testing - New and feature
  • N/A or can only be tested interactively
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• Tooling
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    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode
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    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).
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