Add PowerShell developer workload

[Gijs Reijn (Gijsreyn)]

Adds a new PowerShell workload that bootstraps a PowerShell 7 development environment.

This includes the following:

• Install PowerShell 7 and VS Code.
• Installing Microsoft.VSCode.Dsc from the PowerShell Gallery in the CurrentUser profile.
• Uses this module to install declaratively the PowerShell and Pester extensions in VS Code.
• Configures PSScriptAnalyzer rules in the user settings.

A couple of things to note:

1. The current Microsoft.VSCode.Dsc hasn't officially been released to PowerShell Gallery
   • If in the future Microsoft.PowerShell.PSResourceGet v1.3.0 is released, a recommended location would be MAR (as it it is a MIcrosoft module and MAR will be the default there, increasing security for users)
2. The same applies for the DSC resource shipped with Microsoft.PowerShell.PSResourceGet. See release: https://github.com/PowerShell/PSResourceGet/releases/tag/v1.3.0-preview1