Reconsider `--extra-checks`

--extra-checks was added per this review comment: https://github.com/python/mypy/pull/15425#issuecomment-1604275753

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.

Start by reading the linked review comment and the current handling of --extra-checks and --strict. Compare the proposed --strict-concatenate and --strict-typeddict-update organization with the backwards-compatibility requirement; done means the flag design and any broader bundling decision are documented and agreed.