Properly insert text in document when parsing

If the last child of a node is a Text child and we are inserting text in that node,
we need to append it to that Text child. Doing that means that
HTMLStyleElement::children_changed gets called less frequently, but that's not a
problem during parsing thanks to the pop hook.

components/script/dom/htmltextareaelement.rs

@@ -425,6 +425,13 @@ impl VirtualMethods for HTMLTextAreaElement {
             }
         }
     }
+
+    fn pop(&self) {
+        self.super_type().unwrap().pop();
+
+        // https://html.spec.whatwg.org/multipage/#the-textarea-element:stack-of-open-elements
+        self.reset();
+    }
 }
 
 impl FormControl for HTMLTextAreaElement {}