Set up publishing App Hosting adapters to local registry for buildpacks testing

[blidd-google]

During local development, we may want to iterate on early changes to a build adapter in the context of the buildpacks, without cutting a release to the @apphosting npm registry. This PR enables publishing build adapter prototypes to a local registry (powered by Verdaccio) for development.

Steps:

1. Add an npm user account for the local registry: npm adduser --registry http://localhost:4873
2. Start the local registry: npm run registry:local
3. Publish the adapter: npm run publish:local