add "run on Lean4 Playground" button to code blocks

assets/blockPlay.js

@@ -0,0 +1,23 @@
+Array.from(document.querySelectorAll(".language-lean")).forEach(function (code_block) {
+  let pre_block = code_block.parentElement;
+
+  // create a link to lean4 web editor
+  let escaped_code = encodeURIComponent(code_block.textContent);
+  let url = `https://live.lean-lang.org/#code=${escaped_code}`;
+
+  // create a button
+  let buttons = pre_block.querySelector(".buttons");
+  let leanWebButton = document.createElement('button');
+  leanWebButton.className = 'fa fa-external-link lean-web-button';
+  leanWebButton.hidden = true;
+  leanWebButton.title = 'Run on lean4 playground';
+  leanWebButton.setAttribute('aria-label', leanWebButton.title);
+
+  // insert the button
+  buttons.insertBefore(leanWebButton, buttons.firstChild);
+
+  // open the link when the button is clicked
+  leanWebButton.addEventListener('click', function (e) {
+    open(url);
+  });
+});

book.toml

@@ -6,4 +6,7 @@ src = "md"
 title = "Metaprogramming in Lean 4"
 
 [output.html]
-git-repository-url = "https://github.com/leanprover-community/lean4-metaprogramming-book"
+git-repository-url = "https://github.com/leanprover-community/lean4-metaprogramming-book"
+additional-js = [
+  "assets/blockPlay.js", # Add "Run on Lean4 Playground" button to each code block
+]