WIP: Linux support

[assaron]

Addresses #5

1. Limits number of created RtMidi objects.
2. Adds a basic menu.

[aolsenjazz]

Thanks. Would you add linux to the .github/workflows/test.yml workflow matrix and check passing prior to merging? Prettier is also complaining - it's preconfigured so just need to run prettier in cli.

I remember running into some weirdness re. drivers when running tests in CI environment a while back on linux so may not be non-trivial.

I'll check what's required for linux publishing

[assaron]

OK, the tests are passing now.