Add coverage via nyc

https://github.com/bcoe/nyc

See example setup in https://github.com/bahmutov/node-sentry-error-reporter/blob/master/package.json

Start by reading the linked nyc project and comparing the example setup in node-sentry-error-reporter/package.json with this generator's generated package.json. Identify where the generator assembles package metadata and test scripts. Done means generated projects include nyc-based coverage configuration and the existing tests still run successfully.

Written by the indexing model from the issue text.