feat(generators): add string constant for top-level VSCT GUIDs

[CalvinAllen]

Summary

• Top-level GUIDs (those without IDSymbols) now generate both a string constant and a Guid field
• This matches the behavior of command set GUIDs which already have GuidString and Guid members

Before:

public static readonly Guid guidSamplePackage = new Guid("...");

After:

public const string guidSamplePackageString = "...";
public static readonly Guid guidSamplePackage = new Guid(guidSamplePackageString);

Test plan

• Build generators project
• Verify sample project generates updated VSCT code