Use API Extractor for TypeScript modules

[aslakhellesoy]

Is your feature request related to a problem? Please describe.

Exposing the right API from TypeScript modules is tricky as we've discussed in https://github.com/cucumber/common/pull/1660#pullrequestreview-708109485

Describe the solution you'd like

Adopt API Extractor which solves this problem

Describe alternatives you've considered

Using a homegrown namespace as currently implemented in https://github.com/cucumber/common/pull/1660, but that doesn't really solve the "we forgot to export this file" problem.

Additional context

This is also related to @mpkorstanje's recent soapbox talks about having stable public APIs.

[davidjgoss]

This will be great! I did try to add it a while ago and ran into some issues, but IIRC it was because of some peculiarities in the TypeScript code emitted by protobufjs, so hopefully smooth sailing this time.