Should `--extra-checks` be enabled by default

https://kotlinisland.github.io/basedmypy/command_line.html#cmdoption-mypy-extra-checks

> This flag enables additional checks that are technically correct but may be impractical in real code. In particular, it prohibits partial overlap in `TypedDict` updates, and makes arguments prepended via `Concatenate` positional-only.

Start with the linked --extra-checks command-line documentation and trace where the flag's default is defined in the project. Done means the default decision is documented and, if changed, the command-line behavior and relevant documentation reflect it.

Written by the indexing model from the issue text.