[3.x] [Mono] Add support for file-scoped namespace declaration.

[zaevi]

Support for parsing C# scripts with file-scoped namespace declaration:

using Godot;

namespace MyGame;

public class Main : Node2D
{
    // ...
}

[Zireael07]

Shouldn't this have a master version, too?

[zaevi]

Shouldn't this have a master version, too?

Actually, it's a FIX to customized C# parser for 3.x. Master doesn't need this :P

[raulsntos]

This is C# 10 syntax and the .csproj that we create by default targets net472 (which defaults to C# 7.3) so it wouldn't really be valid but I think it's fine for this parser to consider it valid since it's only used to retrieve the script metadata.

In master this parser no longer exists as it was replaced with source generators (#46713).

[zaevi]

Yes, so I usually switch my csproj target to netstandard2.1 with <LangVersion>preview</LangVersion>.

For preview, some syntactic sugars for C# 8+ are valid, like file-scoped namespace, global using and pattern-matching enhancements😄.

[akien-mga]

Thanks!