reading a class
```ebnf
class ::= class_scope? T_CLASS T_STRING (T_EXTENDS NAMESPACE_NAME)? (T_IMPLEMENTS (NAMESPACE_NAME ',')* NAMESPACE_NAME)? '{' CLASS_BODY '}'
```
function() {
    const result = this.node("class");
    const flag = this.read_class_scope();
    // graceful mode : ignore token & go next
    if (this.token !== this.tok.T_CLASS) {
      this.error(this.tok.T_CLASS);
      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_namespace_name();
    }
    let propImplements = null;
    if (this.token == this.tok.T_IMPLEMENTS) {
      propImplements = this.next().read_name_list();
    }
    this.expect("{");
    const body = this.next().read_class_body();
    return result(propName, propExtends, propImplements, body, flag);
  }