feat: add workday endpoint

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.

README.md

@@ -239,3 +239,15 @@ holidayApi.holidays({
   year: 2019,
 });
 ```
+
+### Workday
+
+#### Fetch workday 7 business days after a date
+
+```javascript
+holidayApi.workday({
+  country: 'US',
+  start: '2019-07-01',
+  days: 7,
+});
+```