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/annotation_layer_builder.js

@@ -123,7 +123,7 @@ class AnnotationLayerBuilder {
       // if there is at least one annotation.
       this.div = document.createElement("div");
       this.div.className = "annotationLayer";
-      this.pageDiv.appendChild(this.div);
+      this.pageDiv.append(this.div);
       parameters.div = this.div;
 
       AnnotationLayer.render(parameters);