Add [Experimental] To [BindableProperty]

[TheCodeTraveler]

Description of Change

This PR adds the [Experimental] To [BindableProperty].

As discussed in our December 2025 Standup, our plan is to release [BindableProperty] in the CommunityToolkit.Maui NuGet Package as an experimental feature that developers are required to opt-in to until we are confident it is ready to be promoted to stable.

As Microsoft notes in its documentation, this attribute allows us to ship preview features in a stable library:

ExperimentalAttribute allows call sites to be flagged with a diagnostic that indicates that an experimental feature is used. You can use this attribute to ship preview features in otherwise stable libraries.

With the new [Experimental] attribute, the this is now the generated code for BindablePropertyAttribute:

// <auto-generated>
// See: CommunityToolkit.Maui.SourceGenerators.Internal.BindablePropertyAttributeSourceGenerator

#pragma warning disable
#nullable enable
namespace CommunityToolkit.Maui;

[global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
[global::System.AttributeUsage(global::System.AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
[global::System.Diagnostics.CodeAnalysis.Experimental("MCTEXP001")]
sealed partial class BindablePropertyAttribute : global::System.Attribute
{
	public string? PropertyName { get; }
	public global::System.Type? DeclaringType { get; set; }
	public object? DefaultValue { get; set; }
	public global::Microsoft.Maui.Controls.BindingMode DefaultBindingMode { get; set; }
	public string ValidateValueMethodName { get; set; } = string.Empty;
	public string PropertyChangedMethodName { get; set; } = string.Empty;
	public string PropertyChangingMethodName { get; set; } = string.Empty;
	public string CoerceValueMethodName { get; set; } = string.Empty;
	public string DefaultValueCreatorMethodName { get; set; } = string.Empty;
}

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard

Additional information

Once we confirm that [BindableProperty] is stable we will remove [Experimental] promoting it to stable.

This PR also adds <NoWarn>MCTEXP001</NoWarn> to Directory.Build.props which enables our libraries to opt-in to the experimental [BindableProperty]. Without this NoWarn, the [Experimental] would generate a compiler error.

This PR also removes <InternalsVisibleTo Include="CommunityToolkit.Maui.SourceGenerators.Internal.UnitTests"/> from Directory.Build.props because it was causing a compiler error due to the fact that PolySharp provides an implementation of System.Diagnostics.CodeAnalysis.ExperimentalAttribute (so that we can use in .NET Standard 2.0 library) which was conflicting a conflicting-types error with the actual .NET 10 implementation of System.Diagnostics.CodeAnalysis.ExperimentalAttribute provided by the BCL in CommunityToolkit.Maui.SourceGenerators.Internal.UnitTests.