Support nesting and generics for union types

[jeffijoe]

Union nesting

This PR adds support for nesting union types within other types.

using System.Collections.Generic;
using FxKit.CompilerServices;

namespace Super.Duper.Unions;

public partial class One
{
    [Union]
    public partial record Problem
    {
        partial record Invalid;
        partial record Denied;
    }
}

public partial class Two
{
    [Union]
    public partial record Problem
    {
        partial record Invalid;
        partial record Denied;
    }
}

EnumMatch generator refactor

The EnumMatch generator has also been refactored for performance by properly utilizing incremental generation. The syntax writer has also been updated to use IndentedTextWriter

MustBePartialAnalyzer - check ancestry

The MustBePartialAnalyzer has been updated to check the type's ancestors for partial.

using FxKit.CompilerServices;

namespace Super.Duper.Unions;

// missing partial
public class Super
{
    // missing partial
    public class Duper
    {
        [Union]
        public partial record Nested
        {
          partial record Incorrect;
        }
    }
}

The Super and Duper types will be reported as missing partial.