Moves duplicate field script into its own file

bookwyrm/static/js/forms.js

@@ -0,0 +1,32 @@
+(function () {
+    "use strict";
+
+    /**
+     * Duplicate an input field
+     *
+     * @param {event} the click even on the associated button
+     */
+    function duplicateInput(event) {
+        const trigger = event.currentTarget;
+        const input_id = trigger.dataset.duplicate;
+        const orig = document.getElementById(input_id);
+        const parent = orig.parentNode;
+        const new_count = parent.querySelectorAll("input").length + 1;
+
+        let input = orig.cloneNode();
+
+        input.id += "-" + new_count;
+        input.value = "";
+
+        let label = parent.querySelector("label").cloneNode();
+
+        label.setAttribute("for", input.id);
+
+        parent.appendChild(label);
+        parent.appendChild(input);
+    }
+
+    document
+        .querySelectorAll("[data-duplicate]")
+        .forEach((node) => node.addEventListener("click", duplicateInput));
+})();