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.

.eslintrc.json

@@ -22,23 +22,37 @@
     "jest"
   ],
   "rules": {
-    "@typescript-eslint/no-unused-vars": "error",
+    "@typescript-eslint/no-unused-vars": [
+      "error",
+      {
+        "vars": "all",
+        "args": "after-used",
+        "ignoreRestSiblings": false
+      }
+    ],
     "import/extensions": [
       "error",
-      "always",
+      "ignorePackages",
       {
-        "ts": "never",
-        "tsx": "never"
+        "ts": "never"
       }
     ],
     "import/no-extraneous-dependencies": [
       "error",
       {
-        "devDependencies": ["**/*.test.ts"]
+        "devDependencies": [
+          "**/*.test.ts"
+        ]
       }
     ],
     "import/prefer-default-export": "off",
-    "lines-between-class-members": "off",
+    "lines-between-class-members": [
+      "error",
+      "always",
+      {
+        "exceptAfterSingleLine": true
+      }
+    ],
     "no-unused-vars": "off"
   }
 }