Support new C# styles

[kekyo]

ImplicitUsing

In .NET 6 SDK cli:

dotnet new console

Source code results only 1 line:

Console.WriteLine("Hello world");

That code is placed into global namespace. IL2C currently doesn't support any global namespaces.

Nullable

Any nullable reference types don't affect any IL code, but it requires to implement by some attribute types.
IL2C currently doesn't have Attribute base type.

In IL2C, it may be better to just define an Attribute type, since (for now) all attributes are simply ignored.

[kekyo]

In 3400c23, found same problem for global function in built net6.0 tfm.
This tfm always created Main() method and assigned for entry point attribute on library assembly.

Related #119