(dsc): Add `_purge` to `PSResourceList` resource

[Gijsreyn]

Summary of the new feature / enhancement

As a user, I want _purge: true on a PSResourceList instance so that any resource installed from that repository and not present in resources is uninstalled during set, and reported as a difference during test.

resources:
- name: Only these modules
  type: Microsoft.PowerShell.PSResourceGet/PSResourceList
  properties:
    repositoryName: PSGallery
    _purge: true
    resources:
    - name: Pester
    - name: PSScriptAnalyzer

With Pester, PSScriptAnalyzer and Az.Accounts installed from PSGallery, test reports _inDesiredState: false and set uninstalls Az.Accounts.

Proposed technical implementation details (optional)

• Add _purge to the PSResourceList schema, referencing the DSC purge.json property schema.
• In GetPSResourceListActions, when _purge is set, append an Uninstall action for every installed resource from the repository that has no matching desired entry. test, set and what-if pick this up automatically since they share that function.
• Respect scope: only purge from the scopes the listed resources use, or from both when none is specified, and document which one was chosen.