Add playground environment for end-to-end testing

[mfogliatto]

Description

This PR adds a self-contained playground environment for end-to-end testing of ReferenceCop changes during development.

The playground provides a fast inner-loop workflow where developers can:

• Build the ReferenceCop package
• Test it against sample projects with various rule configurations
• Validate changes end-to-end without impacting the main solution

Key features:

• Offline dev feed: NuGet config points directly to package build output (../../src/ReferenceCop.Package/bin/Debug)
• Automatic version detection: Package version is written to package-version.txt during pack and automatically read by Directory.Build.props
• Centralized package reference: ReferenceCop referenced via Directory.Build.props for all playground projects
• Example configurations: Includes all three rule types (AssemblyName, ProjectTag, ProjectPath) in ReferenceCop.config

Type of change

• Enhancement (e.g. new feature, functional improvement, etc.)
• Bug fix
• Chore (e.g. refactoring, documentation, etc.)

Related Issue

Related to #43 - This playground environment serves as preparation for automated E2E testing in CI/CD workflows

How Has This Been Tested?

• Manually tested the workflow:
  1. Built ReferenceCop package: dotnet pack -c Debug in src/ReferenceCop.Package
  2. Restored playground project: dotnet restore in playground/TestProject/SampleApp
  3. Built playground: dotnet build in playground/TestProject/SampleApp
• Verified that violations are detected:
  • SampleLibrary includes Newtonsoft.Json reference which triggers the "NoNewtonsoftJson" warning rule
  • Build output shows expected diagnostic message

Screenshots (if applicable)

N/A

Additional context

Structure:

• playground/ - Root directory
  • TestProject/ - Sample projects
    • SampleApp/ - Console app (ProjectTag: App)
    • SampleLibrary/ - Class library (ProjectTag: Library, includes Newtonsoft.Json)
    • Directory.Build.props - Centralized ReferenceCop package reference with auto-version
    • ReferenceCop.config - Example rules for all three rule types
    • nuget.config - Offline feed configuration
  • README.md - Complete workflow documentation

Workflow:

1. Make changes to ReferenceCop source
2. Build package: cd src\ReferenceCop.Package; dotnet pack -c Debug
3. Test: cd playground\TestProject\SampleApp; dotnet restore; dotnet build