feat: Add UnstructuredTSVLoader (#7367)

### Summary

Adds an `UnstructuredTSVLoader` for TSV files. Also updates the doc
strings for `UnstructuredCSV` and `UnstructuredExcel` loaders.

### Testing

```python
from langchain.document_loaders.tsv import UnstructuredTSVLoader

loader = UnstructuredTSVLoader(
    file_path="example_data/mlb_teams_2012.csv", mode="elements"
)
docs = loader.load()
```

tests/integration_tests/document_loaders/test_tsv.py

@@ -0,0 +1,15 @@
+import os
+from pathlib import Path
+
+from langchain.document_loaders import UnstructuredTSVLoader
+
+EXAMPLE_DIRECTORY = file_path = Path(__file__).parent.parent / "examples"
+
+
+def test_unstructured_tsv_loader() -> None:
+    """Test unstructured loader."""
+    file_path = os.path.join(EXAMPLE_DIRECTORY, "stanley-cups.tsv")
+    loader = UnstructuredTSVLoader(str(file_path))
+    docs = loader.load()
+
+    assert len(docs) == 1

tests/integration_tests/examples/stanley-cups.tsv

@@ -0,0 +1,5 @@
+Stanley Cups		
+Team	Location	Stanley Cups
+Blues	STL	1
+Flyers	PHI	2
+Maple Leafs	TOR	13