add copy button to code blocks

lib/custom_html.typ

@@ -12,6 +12,8 @@
   html.elem("theme-picker", attrs: (aria-label: label, role: "radiogroup"), body)
 }
 #let code-block = html.elem.with("code-block")
+#let code-block-button-container = html.elem.with("code-block-button-container")
+
 #let icon-container(..args, body) = {
   html.elem("icon-container", attrs: args.named(), body)
 }