feat: add perfStats to json reporter

[zxch3n]

Add the missing field perfStats to JSON TestResult compared to Jest test result type.

{  
  ...
  perfStats: {
    end?: Milliseconds
    runtime?: Milliseconds
    start?: Milliseconds
  },
  ...
}

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[sheremet-va]

What about slow? Also can we make a snapshot of JSON instead of a string here? 👀

[zxch3n]

slow would require additional project config, like jest's slowTestThreshold [number].

[sheremet-va]

LGTM

[JessicaSachs]

Any reason we're not using performance.now()? My test fixtures are too fast and Date.now() is giving me "0" in the duration of my tasks.

[zxch3n]

@JessicaSachs duration is recorded somewhere else, which is not related to this PR

[JessicaSachs]

That's fine, I meant in general. @sheremet-va and I spoke on Discord and he asked me to make an issue to swap the recording of stats throughout Vitest from Date.now => performance.now, citing incompatibility with previous Node versions. Vitest couldn't import from node:performance at the time, as that would've broken browser support. #6377 for future discussion.