LibWeb: Add capabilities to find the index of a child in its parent.

For Elements depending on the index they are inside their parent. Most
notably the <ol> element.
Also added a typed version to only count children of a certain type.

This patch is work towards #2059

Userland/Libraries/LibWeb/TreeNode.h

@@ -97,6 +97,39 @@ public:
         return const_cast<TreeNode*>(this)->child_at_index(index);
     }
 
+    Optional<size_t> index_of_child(const T& search_child)
+    {
+        VERIFY(search_child.parent() == this);
+        size_t index = 0;
+        auto* child = first_child();
+        VERIFY(child);
+
+        do {
+            if (child == &search_child)
+                return index;
+            index++;
+        } while (child && (child = child->next_sibling()));
+        return {};
+    }
+
+    template<typename ChildType>
+    Optional<size_t> index_of_child(const T& search_child)
+    {
+        VERIFY(search_child.parent() == this);
+        size_t index = 0;
+        auto* child = first_child();
+        VERIFY(child);
+
+        do {
+            if (!is<ChildType>(child))
+                continue;
+            if (child == &search_child)
+                return index;
+            index++;
+        } while (child && (child = child->next_sibling()));
+        return {};
+    }
+
     bool is_ancestor_of(const TreeNode&) const;
     bool is_inclusive_ancestor_of(const TreeNode&) const;
     bool is_descendant_of(const TreeNode&) const;