[code style] Prefer file-scope namespaces

[MartyIX]

Description of Change

This PR adds a suggestion to use file-scoped namespaces (if possible) using IDE0161 analyzer.

The documentation page mentions:

This option [i.e. csharp_style_namespace_declarations] is used by Visual Studio to determine how namespaces are declared when new code files are added to projects. Visual Studio honors the option value even if both IDE0160 and IDE0161 are disabled.

This PR was created because when file-scoped namespaces are used, then there is bigger chance that two files can be worked on one display which leads to improved productivity.

What are file-scoped namespaces?

C# originally allowed only block-scoped namespaces:

using System;

namespace Microsoft.Maui
{
	internal static class ArrayExtensions
	{
	}
}

Modern C# versions support:

using System;

namespace Microsoft.Maui;

internal static class ArrayExtensions
{
}

as it saves one level of indentation, so more code fits on your screen.

[dotnet-policy-service]

Hey there @MartyIX! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[MartyIX]

Suggestion to avoid waaaay too many warnings.

[MartyIX]

Picked up by MSVS when user creates a new type (class, interface, etc.).

[jsuarezruiz]

/azp run

[PureWeen]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).