[Feature]: Support C# 15 union types (AreUnions / BeUnions) #520

Description

C# 15 (.NET 11) adds union types:

public record Cat(string Name);
public record Dog(int Age);

public union Pet(Cat, Dog);

ArchUnitNET already loads these correctly. A union shows up as a Struct with one constructor per case type and a Value property. Dependencies are found both from the union to its case types and from code that switches over the union. The only gap is that rules can't tell a union apart from an ordinary struct.

The compiler marks a union type with:

• [System.Runtime.CompilerServices.UnionAttribute]
• the interface System.Runtime.CompilerServices.IUnion

Proposal:

• an IsUnion property on the type
• the predicates AreUnions() / AreNotUnions() and the conditions BeUnions() / NotBeUnions() on types, next to AreStructs()

I'd be happy to implement this and open a PR if the proposal sounds good to you.

Use Case(s)

// Unions only in the domain model
Types().That().AreUnions().Should().ResideInNamespace("MyApp.Domain");

// Results are unions, not inheritance hierarchies
Types().That().HaveNameEndingWith("Result").Should().BeUnions();