Set Error.prototype.cause type to unknown (#70)

[iamharbie]

Describe your changes
This PR fixes #45167 which changes the type of Error.prototype.cause from Error to Unknown

Testing performed
I added new tests to ensure that unknown type is recognised

Additional context
Could also potentially resolve #48098. @DanielRosenwasser's suggested being more restrictive with the type of the cause property of the options param. I am open to discussions on this.

Fixes #45167

Credits to Bloomberg colleagues who helped with reviews @mkubilayk @dragomirtitian

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[voxpelli]

This is a good change. Would fix voxpelli/pony-cause#35

[voxpelli]

@iamharbie @mkubilayk @dragomirtitian What's needed to get the CLA signed? My guess is that Bloomberg already has a signed CLA, just that @iamharbie hasn't been added to it and thus that the bot doesn't pick it up?

[voxpelli]

Oh, apparently the Bloomberg CLA thing is being worked on by @RyanCavanaugh: #49675 (comment)