Added support for our latest endpoint, which calculates the workday a
given number of business days into the future from the given date for
the given country.
Also:
* perf: updated dependencies
* ci: pinned eslint to 7.2.x due to airbnb incompatibility with 7.3.x.
* ci: tweaked / improved eslint rules a bit.
* refactor: updated types to use `;` and include weekday response.
* docs: added example for new endpoint.
* test: Dropped Node v13.x since it's now past it's expiration.
* test: Migrated from Travis CI to GitHub Actions.
* test: Migrated from Coveralls to CodeCov.
* docs: Dropped over 1.x stuff from the read me.
* perf: Updated dependencies and rebuilt against latest TypeScript.
* feat: Bumped major version number due to Node.js support changing.
