Two small fixes

[taralx]

1. Add missing yarn devDependency
   This means that npm install actually installs everything required to e.g. run tests.
2. Make all handle calls await
   Without this, throws in handle cause an annoying "unhandled exception" abort with no stack trace.

[kwhitley]

Looks great! 👍

[kwhitley]

Thanks @taralx! I started a review to suggest replacing references to yarn with npm run in the package.json scripts (instead of adding yarn as a dev dep), but given that I explicitly (for now at least) want a consistent yarn.lock, I think your solution is better.

Re. the async - I was just lazy ;) I called async on the ones that explicitly needed it to resolve the tests and cut corners on the rest obviously. Thanks for the cleanup!! 🙏