LibWeb/IDB: Implement IDBIndex::count

Author: https://github.com/stelar7
Commit: 852323009f
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4727
Reviewed-by: https://github.com/gmta ✅
Reviewed-by: https://github.com/shannonbooth

Libraries/LibWeb/IndexedDB/Internal/Index.h

@@ -43,6 +43,7 @@ public:
     void clear_records();
     Optional<IndexRecord&> first_in_range(GC::Ref<IDBKeyRange> range);
     GC::ConservativeVector<IndexRecord> first_n_in_range(GC::Ref<IDBKeyRange> range, Optional<WebIDL::UnsignedLong> count);
+    u64 count_records_in_range(GC::Ref<IDBKeyRange> range);
 
     HTML::SerializationRecord referenced_value(IndexRecord const& index_record) const;