Make TypeConverterExtension public

[daltzctr]

https://github.com/dotnet/maui/blob/main/src/Controls/src/Core/Xaml/TypeConversionExtensions.cs

Attempting to make any sort of custom markup extension with MAUI is a good amount of pain, due to the massive amount of reflection based code used to look up properties and mutate. Additionally, MAUI makes necessary XAML utility classes private, so library or application developers can't even use them.

Attempting to create a markup extension like the following is more difficult than it needs to be. Access to TypeConversionExtensions would be extremely useful as now I have to effectively duplicate the type conversions and caching. The fact this class is private is an active footgun to library authors, as they may choose to skip caching and introduce memory leaks by rolling their own type conversion code.

        object IMarkupExtension.ProvideValue(IServiceProvider serviceProvider)
        {
            var idiom = GetIdiom();
            Type targetType = null;
            BindableProperty bp = null;

            if (serviceProvider != null)
            {
                if (serviceProvider.GetService(typeof(IProvideValueTarget)) is IProvideValueTarget target)
                {
                    targetType = target.TargetObject.GetType();
                    bp = target.TargetProperty as BindableProperty;
                }
            }

            if (targetType != null && bp != null)
            {
                var ret = Activator.CreateInstance(targetType);
                ret.GetType().GetProperty(bp.PropertyName).SetValue(Convert.ChangeType(ret, ret.GetType()), idiom);
                return Convert.ChangeType(ret, targetType);
            }
            else
            {
                return idiom;
            }
        }