Add AllowMultiple=true to QueryPropertyAttribute (#5180)

Add AllowMultiple=true to QueryPropertyAttribute (#5180)

In the process of building a demo app (https://github.com/matthewrdev/xamarin-forms-4), I discovered that I could not apply multiple parameters; doing so would cause a compile-time error.

The usage of this property in ShellContent, specifically in ApplyQueryAttributes, indicates that the intended behaviour is to allow users to specify multiple URL arguments that will be applied onto the page. Changing AllowMultiple to true fixes this.

For reference, when AllowMultiple is not explicitly set, it's default value is false. See: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/attributeusage.