Add support for bigint to pretty-format

[msafi]

Summary

I was experimenting with BigInt numbers and noticed that Jest will not print these correctly in diffs. For example, for

const f = {
  a: BigInt(1),
  b: BigInt(2)
}

Jest would print the diff something like

Received: f {a: {}, b: {}}
Expected: f {a: {}, b: {}}

because pretty-format thinks BigInt is an object.

I made BigInts print with an n appended, like they print in the browser and Node so that the above would print something like this instead

Received: f {a: 1n, b: 2n}
Expected: f {a: 2n, b: 1n}

Test plan

I added unit tests that mimic the unit tests for number types.

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[SimenB]

Thanks! I left a couple of inline comments to make CI happy.

Beyond those, can you update the chnagelog?

[jeysal]

Cool! LGTM after addressing Simen's comments

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