Implement SQL writer

It is often useful to be able to generate SQL from the AST. This opens up possibilities for SQL manipulation or translation. The write should recursively walk the AST and write SQL to a writer / output stream.

A secondary benefit of having a writer is that it often simplifies writing unit tests for the parser, since a SQL statement can be parsed and re-written and then the output can be compared with the input (ignoring whitespace differences)

未指定檔案路徑或測試路徑。先定位 AST 定義和 parser 入口點，然後檢查現有的 parser 單元測試。完成的標準是：透過 writer 或輸出串流將解析後的 AST 遞迴寫成 SQL，並撰寫 round-trip 測試，在忽略空白差異的情況下比較解析後的陳述式和重寫後的陳述式。

由索引模型根據 Issue 內容生成。