Enable the unicorn/prefer-dom-node-append ESLint plugin rule

This rule will help enforce slightly shorter code, especially since you can insert multiple elements at once, and according to MDN `Element.append()` is available in all browsers that we currently support.

Please find additional information here:
 - https://developer.mozilla.org/en-US/docs/Web/API/Element/append
 - https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-dom-node-append.md

web/print_utils.js

@@ -25,7 +25,7 @@ function getXfaHtmlForPrinting(printContainer, pdfDocument) {
   for (const xfaPage of xfaHtml.children) {
     const page = document.createElement("div");
     page.className = "xfaPrintedPage";
-    printContainer.appendChild(page);
+    printContainer.append(page);
 
     const builder = new XfaLayerBuilder({
       pageDiv: page,