Add support for multiple external errors (errors.Join)

[kanaksinghal]

Is your feature request related to a problem? Please describe.

eris.ToJSON & eris.ToString doesn't capture wrapped error stacks after joining multiple errors with standard errors.Join. Even though the data is there.

func main() {
	errOne := eris.New("error one")
	errTwo := eris.New("error two")

	errThree := eris.Wrap(errors.Join(errTwo, errOne), "error three")

	fmt.Println(eris.ToString(errThree, true))
}

// Outputs:
// error three
// 	main.main:/tmp/sandbox2016565250/prog.go:14
// error two
// error one

This is because the eris.Unpack expects the the error to always be a linear chain of eris errors ending with one external error. The un-wrap loop breaks upon finding the first "external error". In the above case being described, the Error chain would be something like this: *eris.rootError -> `*errors.joinError{errs: []error{*eris.rootError, *eris.rootError}}.

Describe the solution you'd like

Update the eris.Unpack to expect multiple external errors in the chain of eris errors.

something like this:

// UnpackV2 returns a human-readable UnpackedError type for a given error.
func UnpackV2(err error) UnpackedErrorV2 {
	var upErr UnpackedErrorV2
	for err != nil {
		switch err := err.(type) {
		case *rootError:
			upErr.ErrChain = append(upErr.ErrChain, ErrRoot{
				Msg:  err.msg,
				Stack: err.stack.get(),
			})
		case *wrapError:
			// prepend links in stack trace order
			upErr.ErrChain = append(upErr.ErrChain, ErrLink{
					Msg:  err.msg,
					Frame: err.frame.get(),
			})
		default:
			upErr.ErrChain = append(upErr.ErrChain, ErrExternal(err))
		}
		err = Unwrap(err)
	}
	return upErr
}

type UnpackedErrorV2 struct {
	ErrChain    []interface{
		errType() string
		formatJSON() map[string]interface{}
		formatStr() string
	}
}

Describe alternatives you've considered

I've considered following feature request, although the blast radius seems too big in that so I was trying to imagine a smaller and backward-compatible way of doing this.

#101

Additional context