Added implementation of standard ISO-GQL syntax(3)：source/destination predicate

【需求描述】
新增实现以下标准ISO-GQL语法:
1. source/destination predicate

【期望产出】
1. 支持实现上述语法，同时提供并跑通测试用例。

如有疑问请联系： @puuuuug

The issue names ISO-GQL source/destination predicates but does not identify files, parser entry points, or existing tests. First locate GeaFlow's GQL syntax implementation and related predicate tests, then confirm the expected standard behavior. Done means the syntax is supported and the accompanying tests pass.

Written by the indexing model from the issue text.