Adding dialect labels

I was thinking that it could be nice to add dialect-based labels to the PRs, such as PostgreSQL, SQLite etc, so that reviewers familiar with each of these dialects can appropriately filter PRs. Maybe we can even add an auto-labeller bot.

@alamb @iffyio if you agree I will proceed to do so.

未指定任何文件、测试或入口点。首先明确范围仅限于方言标签，还是也包括自动标注机器人，并就方言列表和标注规则达成一致；当约定的标签存在，且任何约定的自动化都能一致地应用这些标签时，即可视为完成。

由索引模型根据 Issue 内容生成。