Add example using MV3 userScripts API (#576)

userScripts-mv3/userscript_examples/privileged.user.js

@@ -0,0 +1,32 @@
+// ==UserScript==
+// @name          Demo of privileged user script
+// @description   Add button on domains starting with "example" that displays privileged info in a new tab.
+// @include       https://example*
+// @include       http://example*
+// @grant         GM_info
+// @grant         GM_openInTab
+// @version       1.2.3
+// ==/UserScript==
+
+// To test:
+// 1. Visit https://example.com/ or http://example.org/
+// 2. Click on the "Show user script info" button.
+// 2. Confirm that a new tab opens that displays the script info.
+
+/* globals GM_info, GM_openInTab */
+
+if (location.pathname === "/display_userscript_result") {
+  document.body.style.whiteSpace = "pre-wrap";
+  document.body.textContent = decodeURIComponent(location.search.slice(1));
+} else {
+  let button = document.createElement("button");
+  button.textContent = "Show user script info";
+  document.body.prepend(button);
+
+  button.onclick = async () => {
+    let info = await GM_info();
+    let text = `Result from user script:\n\n${JSON.stringify(info, null, 4)}`;
+    let url = "https://example.com/display_userscript_result?" + encodeURIComponent(text);
+    GM_openInTab(url);
+  };
+}