test: add basic tests

Add more tests when mock-fetch is working.

README.md

@@ -18,7 +18,7 @@ or
 
     yarn add spotify-web-playback
 
-Deno or web browser users can use:
+Web browsers can use:
 
 ```javascript
 import Spotify from 'https://esm.sh/spotify-web-playback';

jest.config.js

@@ -0,0 +1,11 @@
+module.exports = {
+  moduleDirectories: ['node_modules', 'src', './'],
+  moduleFileExtensions: ['js', 'json', 'jsx', 'ts', 'tsx'],
+  testEnvironment: 'jsdom',
+  testMatch: ["**/__tests__/**/*.+(ts|tsx|js)", "**/?(*.)+(spec|test).+(ts|tsx|js)"],
+  testURL: 'http://localhost:8989/',
+  verbose: false,
+  transform: {
+    "^.+\\.(ts|tsx)$": "ts-jest",
+  },
+};