Add polyfills for C# 15 union types (UnionAttribute and IUnion)

[AndiJingzhiTse]

Summary

Closes #139

This PR adds source-only polyfills for the two types required by the C# 15 union types feature, enabling their use on downlevel targets before .NET 11 ships these types in the BCL:

• [Union] (System.Runtime.CompilerServices.UnionAttribute) — attribute recognized by the C# 15 compiler to identify a type as a union type
• IUnion (System.Runtime.CompilerServices.IUnion) — interface that union types implement, providing access to the underlying value via object? Value { get; }

Both are placed in the main EmbeddedResources/ folder (language support polyfills, enabled by default) and follow the existing conventions: // <auto-generated/> header, internal accessibility (switchable to public via PolySharpUsePublicAccessibilityForGeneratedTypes), and [ExcludeFromCodeCoverage] on the attribute class.

References

• C# 15 union types blog post
• The blog post notes: "in .NET 11 Preview 2, the UnionAttribute and IUnion interface aren't included in the runtime yet. You must declare them in your project."

Test plan

• Added TestUnion class in tests/PolySharp.Tests/LanguageFeatures.cs that applies [Union] and implements IUnion, exercising both new polyfills
• Build passes: dotnet build — 0 warnings, 0 errors
• All tests pass across all target frameworks (net472, net48, net481, netstandard2.0, netstandard2.1, net8.0, net9.0)

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