reading an interface
```ebnf
interface ::= T_INTERFACE T_STRING (T_EXTENDS (NAMESPACE_NAME ',')* NAMESPACE_NAME)? '{' INTERFACE_BODY '}'
```
function() {
    const result = this.node("interface");
    if (this.token !== this.tok.T_INTERFACE) {
      this.error(this.tok.T_INTERFACE);
      this.next();
      return null;
    }
    this.next().expect(this.tok.T_STRING);
    let propName = this.node("identifier");
    const name = this.text();
    this.next();
    propName = propName(name);
    let propExtends = null;
    if (this.token === this.tok.T_EXTENDS) {
      propExtends = this.next().read_name_list();
    }
    this.expect("{");
    const body = this.next().read_interface_body();
    return result(propName, propExtends, body);
  }