Create a skeleton of DocumentSourceRankFusion stage that registers $rankFusion as an acceptable aggregation stage. 

Since this document source will act as syntactic sugar (like DocumentSourceCount, for example), it only needs to implement a "createFromBson()" method, but doesn't need to implement the DocumentSource interface.

Edit and expand "search_hybrid_scoring_feature_flag.js" to confirm that the aggregation stage is not allowed (and will return an error) when the flag is off. Use "query_stats_feature_flag.js` as an example.