Add SplittingStrategy to the init

Related to #50

Add `SplittingStrategy` to the import statements and `__all__` list in `extract_thinker/__init__.py`.

* Add `from .models.splitting_strategy import SplittingStrategy` to the import statements.
* Add `SplittingStrategy` to the `__all__` list.

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extract_thinker/__init__.py

@@ -15,6 +15,7 @@
 from .text_splitter import TextSplitter
 from .models.classification import Classification
 from .models.contract import Contract
+from .models.splitting_strategy import SplittingStrategy
 
 
 __all__ = [