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

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.

Start with the PSResourceList schema, the DSC purge.json property schema, and GetPSResourceListActions. Trace how test, set, and what-if consume those actions, then verify that unmatched installed resources are reported or uninstalled according to _purge and the selected scope.