PR Summary

Group Policy tools (e.g. gpedit.msc) use administrative template files (.admx, .adml) to populate policy settings in their user interface. This allows administrators to manage registry-based policy settings for PowerShell Core.
This PR:

1. Adds GroupPolicy definitions (.admx, .adml) for PowerShell Core.
2. Adds an installation script that admins can use to install PowerShell Core GroupPolicy definitions.
3. Adds Policy settings Fall-Back functionality described in PowerShell Core Policy RFC.
4. Adds tests for:
   *) registry-based PS Core policy settings;
   *) Fall-Back to Windows PS policy settings functionality.

PR Context

PowerShell Core Policy RFC was recently accepted, so UX related to PowerShell Core GroupPolicy settings has to me adjusted accordingly.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed: Add group policy settings to doc for powershell.config.json #4216
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
  • OR
  • I have considered the user experience from a tooling perspective and enumerated concerns in the summary. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
    • 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).