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.

tests/holidayapi.test.ts

@@ -506,5 +506,106 @@ describe('holidayapi', () => {
         }
       });
     });
+
+    describe('/v1/workday', () => {
+      const basePath = `/workday?key=${key}`;
+
+      it('should return workday', async () => {
+        const expectedResponse = {
+          status: 200,
+          requests: {
+            used: 1000,
+            available: 9000,
+            resets: '2019-10-01 00:00:00',
+          },
+          workday: [
+            {
+              date: '2019-07-16',
+              weekday: {
+                name: 'Tuesday',
+                numeric: '2',
+              },
+            },
+          ],
+        };
+
+        mockRequest.get(`${basePath}&country=US&start=2019-07-01&days=10`)
+          .reply(200, expectedResponse);
+
+        expect(await holidayapi.workday({
+          country: 'US',
+          start: '2019-07-01',
+          days: 10,
+        })).toStrictEqual(expectedResponse);
+      });
+
+      it('should error when country is missing', async () => {
+        expect.assertions(1);
+
+        try {
+          await holidayapi.workday();
+        } catch (err) {
+          expect(err.message).toMatch(/missing country/i);
+        }
+      });
+
+      it('should error when start is missing', async () => {
+        expect.assertions(1);
+
+        try {
+          await holidayapi.workday({ country: 'US' });
+        } catch (err) {
+          expect(err.message).toMatch(/missing start date/i);
+        }
+      });
+
+      it('should error when days is missing', async () => {
+        expect.assertions(1);
+
+        try {
+          await holidayapi.workday({ country: 'US', start: '2019-07-01' });
+        } catch (err) {
+          expect(err.message).toMatch(/missing days/i);
+        }
+      });
+
+      it('should error when days is negative', async () => {
+        expect.assertions(1);
+
+        try {
+          await holidayapi.workday({ country: 'US', start: '2019-07-01', days: -10 });
+        } catch (err) {
+          expect(err.message).toMatch(/days must be 1 or more/i);
+        }
+      });
+
+      it('should raise 4xx errors', async () => {
+        const expectedResponse = {
+          status: 429,
+          error: 'Rate limit exceeded',
+        };
+
+        expect.assertions(1);
+        mockRequest.get(`${basePath}&country=US&start=2019-07-01&days=10`)
+          .reply(429, expectedResponse);
+
+        try {
+          await holidayapi.workday({ country: 'US', start: '2019-07-01', days: 10 });
+        } catch (err) {
+          expect(err.message).toMatch(/rate limit exceeded/i);
+        }
+      });
+
+      it('should raise 5xx errors', async () => {
+        expect.assertions(1);
+        mockRequest.get(`${basePath}&country=US&start=2019-07-01&days=10`).reply(500);
+
+        try {
+          await holidayapi.workday({ country: 'US', start: '2019-07-01', days: 10 });
+        } catch (err) {
+          expect(err.message).toMatch(/internal server error/i);
+        }
+      });
+    });
   });
 });