snowflake column identifier for copy into / query data staged in files

According to documentation, Snowflake allows:
* [query data staged in files](https://docs.snowflake.com/en/user-guide/querying-stage.html#required-parameters) with an identifier of a form `[.]$[.]`
* [copy into table](https://docs.snowflake.com/en/user-guide/data-load-transform.html) with an column identifier containing `$` character like `select t.$1,t.$2,t.$3 from @~/datafile.csv.gz t;`

Although the feature seems to be snowflake specific, it cannot be implemented within snowflake dialect. It's because tokens starting with `$` character are tokenized into `Token::Placeholder`. That's why parser's change is required.

Initial OpenLineage project issue: https://github.com/OpenLineage/OpenLineage/issues/814

从 lexer 和 parser 入手，检查以 `$` 开头的 token 的处理，特别是当前生成 `Token::Placeholder` 的路径，并审查 Snowflake 方言的处理。只要文档中所述的 staged-file 和 COPY INTO 列形式能够正确解析，且不会导致现有占位符语法回归，工作就完成了。

由索引模型根据 Issue 内容生成。