Add a demo test page for the main readme-example.

The demo isn't perfect, since it's compiled before being served, but it's a start.

script/dev.js

@@ -5,5 +5,6 @@ barelyServe({
   outDir: "./.temp/dev",
   esbuildOptions: {
     target: "es5",
+    splitting: false,
   },
 });

src/demo/readme-examples/main.html

@@ -0,0 +1 @@
+<script src="./main.js" href="./main.ts" type="module"></script>

src/demo/readme-examples/main.ts

@@ -0,0 +1,18 @@
+import * as clipboard from "../../clipboard-polyfill/entries/es6/clipboard-polyfill.es6";
+
+function handler() {
+  clipboard.writeText("This text is plain.").then(
+    () => {
+      console.log("success!");
+    },
+    () => {
+      console.log("error!");
+    },
+  );
+}
+
+window.addEventListener("DOMContentLoaded", function () {
+  var button = document.body.appendChild(document.createElement("button"));
+  button.textContent = "Copy";
+  button.addEventListener("click", handler);
+});