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.

该 issue 没有指出任何文件或测试，因此首先梳理现有的数据库专用字段、feature-gating 设置，以及这里和另一个 issue 中提到的 struct-builder 工作。然后将提案收窄到一个具体的 builder 或 gating 里程碑，并定义测试，以证明受支持的数据库功能仍可构建，而无需进行不相关的语法更改。

由索引模型根据 Issue 内容生成。