LibWeb/IDB: Implement IDBIndex::get

Author: https://github.com/stelar7
Commit: https://github.com/LadybirdBrowser/ladybird/commit/e74e571b561
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.cpp

@@ -76,4 +76,11 @@ void Index::clear_records()
     m_records.clear();
 }
 
+Optional<IndexRecord&> Index::first_in_range(GC::Ref<IDBKeyRange> range)
+{
+    return m_records.first_matching([&](auto const& record) {
+        return range->is_in_range(record.key);
+    });
+}
+
 }