Comment handling

I've been working on [a SQL formatter](https://github.com/maxcountryman/forma) that leverages this library extensively. One problem with using `sqlparser-rs` right now is that comments are treated like whitespace and removed from the AST. I'm curious if there would be any interest in bringing comments into the AST? Perhaps this could be optional, since I imagine it's not terribly useful for most cases.

首先檢查 sqlparser-rs 的 lexer、parser 和 AST 處理，以確定目前如何丟棄註解，以及如何適配可選的註解保留。完成標準是確定在 AST 中以 opt-in 方式表示註解的設計，且其行為適用於連結的 SQL formatter；該 issue 未指明檔案或測試。

由索引模型根據 Issue 內容生成。