Reconsider --extra-checks

[hauntsaninja]

--extra-checks was added per this review comment: #15425 (comment)

I think a big advantage of mypy relative to other type checkers is that it is relatively configurable. mypy's always had a number of config flags and the error code system that we've been encouraging allows users to enable and disable errors with granularity.

--extra-checks is not self-descriptive, it's not immediately obvious what it turns on or off. If I was a user and ran into a false positive/negative, I would not be able to connect it with the option. It's also part of --strict, so it's not even that extra — most users serious enough about type checking to know what the flag does will already have it on. I vote we keep --strict-concatenate and add --strict-typeddict-update. We can keep both of these as part of --strict and keep --extra-checks for backwards compatibility.

If we want to do more bundling of flags / error codes, I feel we should generalise --strict.

[sobolevn]

I agree, let's do that for mypy@2.0