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/struct_tree_layer_builder.js

@@ -128,7 +128,7 @@ class StructTreeLayerBuilder {
         this._setAttributes(node.children[0], element);
       } else {
         for (const kid of node.children) {
-          element.appendChild(this._walk(kid));
+          element.append(this._walk(kid));
         }
       }
     }