Support both mysql2 and mysql2/promise in MysqlDialect.

[igalklebanov]

Hey 👋

We support mysql2, not mysql2/promise.

This can be a real pain and waste of time for people setting up kysely in their projects.

You setup mysql/promise pass it's pool stuff to the dialect, it doesn't yell at you. You spin up your service locally, and queries just hang, no errors, no indication what's wrong. A needle in a haystack.

Can we detect, on driver initialization, whether it's the promise pool or the callback pool, and apply the right strategy from that point on?

[koskimas]

Makes sense. We can just promisify the callback version very early on in the dialect so that most of the dialect code can use just one API.