core[minor], community[patch], langchain[patch]: move BaseChatLoader to core (#19607)

Thank you for contributing to LangChain!

- [ ] **PR title**: "core: move BaseChatLoader and BaseToolkit from
community"


- [ ] **PR message**: move BaseChatLoader and BaseToolkit

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/community/langchain_community/chat_loaders/slack.py

@@ -5,11 +5,10 @@ import zipfile
 from pathlib import Path
 from typing import Dict, Iterator, List, Union
 
+from langchain_core.chat_loaders import BaseChatLoader
 from langchain_core.chat_sessions import ChatSession
 from langchain_core.messages import AIMessage, HumanMessage
 
-from langchain_community.chat_loaders.base import BaseChatLoader
-
 logger = logging.getLogger(__name__)