Transform GraphQL Source Generator into Reusable NuGet Package

This PR transforms the GraphQL source generator project into a fully-featured, reusable NuGet package with comprehensive configuration options, error handling, and documentation.

Changes

Package Configuration

• Added package metadata to GraphQLSourceGen.csproj
• Created LICENSE file with MIT license
• Added CHANGELOG.md for version tracking
• Configured proper assembly references

Configuration System

• Created GraphQLSourceGenOptions.cs for configuration options
• Added MSBuild property support via GraphQLSourceGen.props
• Implemented customisable settings:
  • Output namespace
  • Records vs classes
  • Init-only vs mutable properties
  • XML documentation comments

Error Handling

• Added diagnostic descriptors with error codes
• Implemented validation for fragment names and references
• Added clear, actionable error messages
• Improved error reporting in the generator

Code Generation Improvements

• Added nullable reference type support
• Fixed nested type generation to avoid naming conflicts
• Improved XML documentation in generated code
• Added support for proper C# type mapping

Sample Project

• Created GraphQLSourceGen.Samples project
• Added sample GraphQL schema with various fragment types
• Implemented demo program showing usage of generated types
• Reorganised folder structure for better maintainability

CI/CD Pipeline

• Added GitHub Actions workflow for build and test
• Created workflow for publishing to NuGet on release

Documentation

• Updated README with comprehensive installation and usage guides
• Added configuration documentation
• Included troubleshooting section
• Added examples for different GraphQL fragment types

Testing

The changes have been tested by:

• Building the solution with the new structure
• Verifying the sample project works correctly
• Checking that the generated code compiles and runs