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.

首先检查 sqlparser-rs 的 lexer、parser 和 AST 处理，以确定当前如何丢弃注释，以及如何适配可选的注释保留。完成标准是确定在 AST 中以 opt-in 方式表示注释的设计，并且其行为适用于链接的 SQL formatter；该 issue 未指明文件或测试。

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