* Add 'ImplicitUsings=true' to all the templates.
* Make the implicit global namespaces C#-only.
* Add the implicit global namespaces to the 'Using' itemgroup instead of the
'Import' itemgroup.
* Make sure the global namespaces are set from AutoImport.props, so that the
user may remove any global namespace they don't want in their project file
(by doing something like: `<Using Remove="Foundation" />`)
Ref: https://github.com/dotnet/sdk/issues/19521
Ref: https://github.com/dotnet/sdk/issues/19793
Fixes https://github.com/xamarin/xamarin-macios/issues/12457.