WIP: Move to ESM

[hughrawlinson]

This isn't working unfortunately, because Jest's support for ESM codebases without transforming to commonjs is experimental, and I don't feel enthusiastic about bringing babel in. Right now the issue is that it's failing to mock the osc module. I tried using unsafe_mockModule and no the tests failed with saying that the port is already in use, so evidently it didn't mock it.