Implement Jest setup for unit testing class interfaces

[towerofnix]

We're going to be doing a sizeable amount of interface reworking in the near future, and it would be nice to verify compatibility with existing Leopard dependants where possible (if only to make releasing a 2.0.0 which drops all deprecated interfaces at once more convenient, rather than bumping a major version for each dropped interface).

Preparing unit tests lets us ensure deprecated interfaces keep working until formally dropped, and makes room to help ensure we don't break the behavior of any new interfaces either!

[towerofnix]

This didn't turn out to be relevant for #177 in the end - I do think unit tests are worth investigating but I'm going to slide it off my plate and back into the serving pot before I take a bite 🍝