Add flag to raise error if match statement does not match exaustively

[Don-Burns]

Fixes #19136

Change is to add a mode to catch when a match statement is not handling all cases exhaustively, similar to what pyright does by default.
After discussion on #19136 I put it behind a new flag that is not enabled by default.
I updated docs to include information on the new flag also.

Please let me know if anything is not following standards, in particular I wasn't sure what to name this new flag to be descriptive while following existing flag naming style.

[A5rocks]

1. could you enable it by default for mypy primer's output
2. IMO it would be neater to put this behind a new error code instead. Though those are even less discoverable than new modes so...

[Don-Burns]

1. could you enable it by default for mypy primer's output
2. IMO it would be neater to put this behind a new error code instead. Though those are even less discoverable than new modes so...

I added where it looked like made sense to pass the arg to mypy_primer (workflow files?), let me know if I misunderstood where to pass this.

I'll have a look at the error code option when I have a chance and see how it feels. Should still be possible to enable discovery, but right now at least I think CLI mention in --help would be ideal so users don't need the docs unless they are really deep diving. Not sure if there is some middle ground in this regard.

[A5rocks]

Sorry I meant specifically just default to enabled for a commit and let mypy primer run.

Anyways, this should already work for an error code, I just don't know how to disable them by default. (The fact it would be a smaller change is why I said it would be neater)

[Don-Burns]

Sorry I meant specifically just default to enabled for a commit and let mypy primer run.

CI run with default set to True
commit: 0f9ed6a
mypy_primer comment: #19144 (comment)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[Don-Burns]

Test passing now.

My preference is for the CLI arg for discoverability in cli help etc.
But if using an error code only to enable via config is preferred, this is what that would look like from what I can tell: https://github.com/python/mypy/compare/master...Don-Burns:mypy:feat/exhaustive-match-error-code-approach?expand=1