Reads the namespace type declaration
```ebnf
use_type ::= (T_FUNCTION | T_CONST)?
```
@see https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y#L335
@return {String|null} Possible values : function, const
function() {
    if (this.token === this.tok.T_FUNCTION) {
      this.next();
      return this.ast.useitem.TYPE_FUNCTION;
    } else if (this.token === this.tok.T_CONST) {
      this.next();
      return this.ast.useitem.TYPE_CONST;
    }
    return null;
  }