Fix adding what-if capability to resources

[SteveL-MSFT]

PR Summary

There's 2 ways to detect if a resource supports what-if:

1. set or delete has the whatIfArg arg
2. resource has the old whatIf operation

Changed the code from using a Vec to using a HashSet to avoid having to check if what-if was already added to capabiltiies.

PR Context

Fix #1501

[Copilot]

Pull request overview

This PR fixes resource capability detection so dsc resource list correctly reports the whatIf capability when a resource supports what-if mode via either the newer whatIfArg mechanism on set/delete or the legacy whatIf operation.

Changes:

• Detect whatIf capability from set.args / delete.args containing whatIfArg, and from the legacy whatIf operation in the manifest.
• Switch capability accumulation in discovery from Vec to HashSet to avoid duplicates.
• Add/extend tests and update the table capability legend to include w for what-if.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
lib/dsc-lib/src/dscresources/dscresource.rs	Makes Capability hashable/equatable to support set-based capability accumulation.
lib/dsc-lib/src/discovery/command_discovery.rs	Implements the updated what-if capability detection logic while parsing manifests.
dsc/tests/dsc_resource_list.tests.ps1	Adds a Pester test intended to validate what-if capability reporting in dsc resource list.
dsc/src/subcommand.rs	Updates table capability display mapping to include WhatIf as w.