Refactor hello-web example for possible future automated testing

For now there's a Node.js test that works, but we're not actually running it in CI,
and Node is not a dependency of the project.

examples/hello-web/test-node.js

@@ -0,0 +1,25 @@
+/**
+ * Node.js test file for hello-web example
+ * We are not running this in CI currently, and Node.js is not a Roc dependency.
+ * But if you happen to have it, you can run this.
+ */
+
+// Node doesn't have the fetch API
+const fs = require("fs/promises");
+global.fetch = (filename) =>
+  fs.readFile(filename).then((buffer) => ({
+    arrayBuffer() {
+      return buffer;
+    },
+  }));
+
+const { roc_web_platform_run } = require("./platform/host");
+
+roc_web_platform_run("./hello-world.wasm", (string_from_roc) => {
+  const expected = "Hello, World!";
+  if (string_from_roc !== expected) {
+    console.error(`Expected "${expected}", but got "${string_from_roc}"`);
+    process.exit(1);
+  }
+  console.log("OK");
+});