OnConflict implementation for SQLite provider uses wrong feature of SQLite

该 issue 没有列出文件或测试；首先定位 SQLite provider 当前的 OnConflict 实现，并阅读链接的 SQLite ON CONFLICT 和 UPSERT 文档。将其行为与其他受支持 provider 的行为以及报告的 NOT NULL 情况进行比较。当选定的 SQLite 行为或其文档化的警告得到共识并完成实现，同时考虑了向后兼容性时，即视为完成。

Describe the bug
I think that implementation of OnConflict uses wrong feature of SQLite.
If you check the SQLite docs mentioned in https://fsprojects.github.io/SQLProvider/core/crud.html#OnConflict you'll see that "ON CONFLICT clause applies to UNIQUE, NOT NULL, CHECK, and PRIMARY KEY constraints".
SQLite has another feature, specifically for UPSERTs - https://sqlite.org/lang_upsert.html . Docs for that feature mention that "UPSERT in SQLite follows the syntax established by PostgreSQL, with generalizations."

I don't know why this feature of SQLite was chosen back in the day. Even though ON CONFLICT was available in SQLite for decades and UPSERT was only added at 2018, when this feature was added to SQLProvider UPSERT had already been available.
Nevertheless, I think it's quite confusing. Behavior of this feature for SQLite is different from other supported providers. I suppose we have two options:

1. Fix SQLProvider implementation to use proper UPSERTS for SQLite, probably adding a configuration toggle somewhere to enable backward compatibility.
2. Add a warning to the docs stating that usage of OnConflict feature for SQLite has a broader scope than one could expect.

In my case, I set OnConflict.Ignore to one of my tables, expecting it to only deal with primary key violations.
In the meantime, I had an error in my app, one of the non-null column hasn't been filled, but SQLite hasn't complained about it and just silently refused to insert a row into a table. At the end, by debugging raw sql I was able to figure things out.