XAML xmlns simplifications

[StephaneDelcroix]

This is a tracker issue for various xmlns related changes.

• protect Maui XmlnsDefinition to avoid overloading it and causing conflicts
• allow XmlnsDefinition in current assembly. This will allow consuming an XmlnsDefinition in the assembly where it is defined.
• support XmlnsCompatibleWith (or other name) attribute to allow aggregating xmlns
• global xmlns: add a GlobalXmlns.cs in the template as a place for users to aggregate xmlns in a new global namespace.
• implicit xmlns: sourcegen some predefined xmlns in a default xmlns (to allow backward compatibility without updating the app template) (opt-out)
• make the declaration of default xmlns implicit (opt-in)
• allow using xmlns prefixes without having to declare them (opt-in)

with those changes, this is the maui XAML of 2025

<ContentPage>
    <StackLayout>
        <MyControl x:Name="control" />
        <Label />
        <sf:Button x:Name="disambiguated" />
    </StackLayout>
</ContentPage>

this set of spec replaces #3376

Please do not comment on individual specs here, but on their own pages. thanks

closes #3376