Update C# templates to introduce file scoped namespaces

[haonanttt]

Purpose

Regarding to #3329, creating this PR to introduce file scoped namespaces.

What have changed

For all .cs files inside dev/VSIX/ItemTemplates and dev/VSIX/ProjectTemplates, the template code format have been changed to file scoped namespaces:

namespace MyNamespace;

public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        ...
    }
}

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[haonanttt]

1 more thing to be confirmed:
Should the .cs files inside dev/VSIX/Extension and dev/VSIX/Shared also be changed?

[michael-hawker]

1 more thing to be confirmed: Should the .cs files inside dev/VSIX/Extension and dev/VSIX/Shared also be changed?

I'm not as familiar with those, but it doesn't hurt to apply this change to every C# file across the entire repo.

File-scoped namespaces were added for .NET 6, which should be everything here.

[michael-hawker]

🦙❤️ Awesome, thanks @haonanttt for putting this together! 🎉🎉🎉 Couple of suggestions to improve partial class coverage for AOT and source generators.

I thought I remembered another small thing related to templates, but can't find it in my chat history. Nothing came to mind when going through the review. @niels9001 want to do a quick pass to see if I missed anything?

[niels9001]

@haonanttt Love these changes 🚀! Since we are removing the Button click event, we should remove it from the Cpp templates as well?

With .NET9 WPF now comes with modern theming and has Mica enabled as default maybe this is something we should enable as well? Especially since this is something that is a default design pattern across Windows, and are just a 3 lines of XAML.

[niels9001]

Don't forget to remove this from the C++ templates as well

[niels9001]

Suggested change

	<Window.SystemBackdrop>
	<MicaBackdrop />
	</Window.SystemBackdrop>

To enable Mica.

[niels9001]

Suggested change

	<Window.SystemBackdrop>
	<MicaBackdrop />
	</Window.SystemBackdrop>

To enable Mica.

[michael-hawker]

@niels9001 good call out! This may have been what I was thinking about unconsciously. It was pretty jarring to see the difference with the new WPF template/theming enabled and the WinUI window just being a black box during our .NET Conf demo: