code action to baseline an individual error

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use case: you're moving a bunch of code around and you want to rebaseline individual errors that have resurfaced but you don't want to baseline any new errors that come up

another use case is probably #293

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alternatively if we decide not to support #293 as a use case of baselining, perhaps this shouldn't appear if the baseline file doesn't already contain any errors of that type, as it suggests that the user is incorrectly using the feature (see here)