Opt-in feature flag to detect UPDATE/DELETE without WHERE

[Expurple]

Today, I was bitten by this kind of bug once again. I think, it would be nice to detect this automatically. I can imagine an opt-in feature flag like require-where. It would inspect UpdateStatement/DeleteStatement before executing and return an error or panic with a message like this:

caught `UPDATE` without `WHERE`! If this is intentional, add a condition like `WHERE 1 = 1`. You can disable these checks by compiling SeaORM without `require-where` feature.

Maybe I'll take a shot at implementing this later.