Add a new `observeRunsFromConfig` experiment, which allows a test run to be observed by a function installed through an `ava.config.*` file.
The function has access to AVA's internal events, which can then be used to report to a file.
AVA's internal event structure is not currently covered by any SemVer guarantees, which is why this feature requires the experimental opt-in.
Does not currently support watch mode. Only the first run is observed.
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