LibWeb: Const qualify HTMLCollection::length

Author: https://github.com/shannonbooth
Commit: 15944c5b26
Pull-request: https://github.com/SerenityOS/serenity/pull/20656
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/DOM/HTMLCollection.h

@@ -38,7 +38,7 @@ public:
 
     virtual ~HTMLCollection() override;
 
-    size_t length();
+    size_t length() const;
     Element* item(size_t index) const;
     Element* named_item(DeprecatedFlyString const& name) const;