better ResearchLens(update that proj later) currently. updated build_rag_prompt and using it in llm_interface now. updated streamlit_app to display chunks better and limit the number of words in the answer, avoiding long chunks in answer.

added bm25_retriever and hybrid_retriever[hybrid search (FAISS + BM25)] and modified the streamlit_app. extending from the previous project - ResearchLens.