Added bilibili loader (#2673) (#2724)

I've added a bilibili loader, bilibili is a very active video site in
China and I think we need this loader.

Example:
```python
from langchain.document_loaders.bilibili import BiliBiliLoader

loader = BiliBiliLoader(
       ["https://www.bilibili.com/video/BV1xt411o7Xu/",
       "https://www.bilibili.com/video/av330407025/"]
)
docs = loader.load()
```

Co-authored-by: 了空 <568250549@qq.com>

tests/integration_tests/document_loaders/test_bilibili.py

@@ -0,0 +1,20 @@
+from langchain.document_loaders.bilibili import BiliBiliLoader
+
+
+def test_bilibili_loader() -> None:
+    """Test Bilibili Loader."""
+    loader = BiliBiliLoader(
+        [
+            "https://www.bilibili.com/video/BV1xt411o7Xu/",
+            "https://www.bilibili.com/video/av330407025/",
+        ]
+    )
+    docs = loader.load()
+
+    assert len(docs) == 2
+
+    assert len(docs[0].page_content) > 0
+    assert docs[1].metadata["owner"]["mid"] == 398095160
+
+    assert docs[1].page_content == ""
+    assert docs[1].metadata["owner"]["mid"] == 398095160