Global xmlns in template

[StephaneDelcroix]

Description

Add a GlobalXmlns.cs in the maui template for users to start filling clr-namespaces in the global namespace

depending on the choice made in #28843 and the uri we pick for the global xmlns, the GlobalXmlns.cs could look like

// some comments about the global xmlns, what's already in it, how to opt-out the implicit (see #28847 )
[assembly: XmlnsDefinition("http://schemas.microsoft.com/dotnet/maui/global", "[default app clr-namespace]")]

// commented out examples on how to add more clr-namespaces and other xmlns

Public API Changes

none

Intended Use-Case

/

[mrlacey]

What's the purpose of adding this capability?
If it's to avoid needing to specify the XMLNS attributes at the root of each file (x-ref #3376) how will the issue of creating XAML files that are not valid XML be addresed?

[StephaneDelcroix]

@mrlacey the full epic is #28836.

the purpose of this specific spec is to propose a single place for registering xmlns in the global namespace.

the spec about making xmlns declaration implicit is at #28849 and #28850. And it's still valid XML.