[C# 15-Fundamentals and reference]: New Feature - unions

### .NET 11 Preview

.NET 11 Preview 3

### Link to speclet

- [Speclet](https://github.com/dotnet/csharplang/blob/main/proposals/unions.md)

### Link to Pull request

- [PR](https://github.com/dotnet/roslyn/pull/82640)

### New behavior

The `union` keyword creates a new type: a union where the member `Value` is an instance of exactly one of the declared union members.

### Language reference tasks

- The `union` keyword defines a new type that is lowered as a `record struct`. A `union` type holds an instance of one of its members.
- Update pattern matching to discuss exhaustiveness and subsumptiom related to unions.
- Update casts to include union casts (implicit and explicit)

### Everyday C# tasks

- Add information about unions for "one of" and "Result" style scenarios.

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[Associated WorkItem - 562440](https://dev.azure.com/msft-skilling/Content/_workitems/edit/562440)