Handle static methods marked as commands.

[mayuki]

This PR enables Cocona to handle static methods that are marked as commands.

class Program
{
    [Command]
    public static void Foo()
        => Console.WriteLine("Foo");
}

Limitation

• The method needs to be explicitly marked as a command. ([Command])
• The type containing the method must be not static class. (static class is compiled as abstract class).