Update WhatsAppChatLoader regex to handle multiple date-time formats (#4186)

This PR updates the `message_line_regex` used by `WhatsAppChatLoader` to
support different date-time formats used in WhatsApp chat exports;
resolves #4153.

The new regex handles the following input formats:
```terminal
[05.05.23, 15:48:11] James: Hi here
[11/8/21, 9:41:32 AM] User name: Message 123
1/23/23, 3:19 AM - User 2: Bye!
1/23/23, 3:22_AM - User 1: And let me know if anything changes
```

Tests have been added to verify that the loader works correctly with all
formats.

tests/integration_tests/document_loaders/test_whatsapp_chat.py

@@ -0,0 +1,19 @@
+from pathlib import Path
+
+from langchain.document_loaders import WhatsAppChatLoader
+
+
+def test_whatsapp_chat_loader() -> None:
+    """Test WhatsAppChatLoader."""
+    file_path = Path(__file__).parent.parent / "examples" / "whatsapp_chat.txt"
+    loader = WhatsAppChatLoader(str(file_path))
+    docs = loader.load()
+
+    assert len(docs) == 1
+    assert docs[0].metadata["source"] == str(file_path)
+    assert docs[0].page_content == (
+        "James on 05.05.23, 15:48:11: Hi here\n\n"
+        "User name on 11/8/21, 9:41:32 AM: Message 123\n\n"
+        "User 2 on 1/23/23, 3:19 AM: Bye!\n\n"
+        "User 1 on 1/23/23, 3:22_AM: And let me know if anything changes\n\n"
+    )

tests/integration_tests/examples/whatsapp_chat.txt

@@ -0,0 +1,4 @@
+[05.05.23, 15:48:11] James: Hi here
+[11/8/21, 9:41:32 AM] User name: Message 123
+1/23/23, 3:19 AM - User 2: Bye!
+1/23/23, 3:22_AM - User 1: And let me know if anything changes