feat: allow defining symbol error extensions

[n1ru4l]

Using symbol keys is a useful approach when composing error handlers as symbol keys are omitted when the error is finally stringified.

It allows middlewares to read extensions without having to assign them to undefined afterwards or using the delete keyword.

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[benjie]

Argh; we can't have it because typescript 4.3 doesn't support symbols so it fails to pass CI. I think I must not have checked that CI had passed. Rolling back.