Namespace AST node.
EBNF grammar:
namespace-declaration:
"namespace" qualified-identifier namespace-body (";")?
qualified-identifier:
identifier ( "." identifier )*
namespace-body:
"{" (using-directives)? (namespace-member-declarations)? "}"
using-directives:
(using-directive)+
namespace-member-declarations:
(namespace-member-declaration)+
namespace-member-declaration:
namespace-declaration
type-declaration
type-declaration:
class-declaration
struct-declaration
interface-declaration
enum-declaration
delegate-declaration