Added API tests + added vitest.config.ts + added fallback of window.fetch with xmlhttprequest

vitest.config.ts

@@ -0,0 +1,16 @@
+/// <reference types="vitest" />
+import { defineConfig } from 'vite';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: './src/test/setup.ts',
+    // you might want to disable it, if you don't have tests that rely on CSS
+    // since parsing CSS is slow
+    css: false,
+    deps: {
+      registerNodeLoader: false
+    }
+  }
+});