Add header to code block

[laurentkempe]

From @meziantou

function copyCodeBlock(elt, id) {
    navigator.clipboard.writeText(document.getElementById(id).textContent)
        .then(() => {
            elt.disabled = true;
            var body = elt.innerHTML;
            elt.innerHTML = "copied";
            setTimeout(() => {
                elt.innerHTML = body;
                elt.disabled = false;
            }, 1000)
        });
}