TypeScript types are wrong when this package is used in an ESM environment

[kaiyoma]

Since this package is CommonJS, we have to use RcSteps.default in our code when importing to an ESM environment, but the types don't recognize this as being possible.

[kaiyoma]

I ended up fixing this by migrating to Vitest (from Jest), which has much better support for using CJS libraries in ESM code.