Database features and API stability

Each new release breaks build, so using the library is like sitting on a time ticking bomb. It's fine when you need to parse literally any SQL query, e.g. you have wild mix of SQLite, MySQL, DuckDB and PostgreSQL commands, but it's rarely needed this way.

It would be better if each database-specific field will be feature-gated, as new syntax added doesn't affect previously added features.

Here struct builders mentioned in another issue would be greatly appreciated.

I would like to contribute into builders part.

該 issue 沒有指出任何檔案或測試，因此首先整理現有的資料庫專用欄位、feature-gating 設定，以及這裡和另一個 issue 中提到的 struct-builder 工作。然後將提案限縮到一個具體的 builder 或 gating 里程碑，並定義測試，以證明受支援的資料庫功能仍可建置，而無需進行不相關的語法變更。

由索引模型根據 Issue 內容生成。