Closes #621

This PR introduces a new DependencyPropertyGenerator component that utilizes source generators on partial properties to generate DependencyProperty quickly in your projects. This is the final API shape available with this PR:

namespace CommunityToolkit.WinUI;

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public sealed class GeneratedDependencyPropertyAttribute : Attribute
{
    public object? DefaultValue { get; init; }

    [DisallowNull]
    public string? DefaultValueCallback { get; init; }

    public bool IsLocalCacheEnabled { get; init; }

    [DisallowNull]
    public Type? PropertyType { get; init; }
}

public sealed class GeneratedDependencyProperty
{
    public const object UnsetValue;
}

This went through some community discussion before the current API design was finalized.

This PR includes:

• The thin API surface with the attribute
• A source generator for the attribute
• A comprehensive set of analyzers
• A code fixer to easily update all trivially supported dependency projects in a project/solution