Better Custom Markup Extension Support

[mikernet]

I'm not sure what the state of this is in MAUI, but it would be suuuuuuuuuper incredible if custom markup extensions could handle more advanced scenarios than just accepting a list of parameters into a constructor. XAML tends to be very verbose, and more powerful custom markup extensions would be a great way to fix many of those issues.

WPF/Xamarin custom markup extensions have to follow a strict syntax like {m:SomeEx value1, value2, value3}. This makes it impossible to write a custom markup extension that follows a different syntax like say {x:MethodBinding Add(!{Binding BoolValue}, {Binding OtherValue}). This severely limits the creative possibilities for some very useful markup extensions.