Add backwards compatibility for older versions of Node

[PaulAsjes]

Description

workos-node v6 added support for other JS runtimes, but that meant that the minimum node version required jumped up to v19.

This adds in backwards compatibility to make the SDK usable with at least Node v16.

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[ ] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.