Add type hints, docstrings, and improve code quality

[Copilot]

Modernize codebase with type annotations and documentation to improve maintainability and IDE support.

Changes

• Type hints: Added comprehensive annotations to all methods (typing.Optional, typing.List, return types)
• Docstrings: Added Google-style docstrings with Args/Returns/Raises sections to all public methods
• Constants: Extracted magic strings to named constants (SENTENCE_SPLIT_MARKER, NER_TAG_BEGIN, NER_TAG_OUTSIDE)
• Variable naming: Replaced underscore-prefixed names with descriptive identifiers (_pos_tag → pos_tags, _new_cut → new_tokens, _ner_ → ner_tags)
• PEP 8: Fixed spacing around type hints, operators, and list comprehensions
• Package structure: Created proper __init__.py with module docstring and exports

Example

Before:

def _pos(self, doc:Doc):
    _pos_tag = []
    for i,_ in enumerate(_pos_tag):
        doc[i].pos_ = _pos_tag[i]

After:

def _pos(self, doc: Doc) -> Doc:
    """Add part-of-speech tags to tokens."""
    pos_tags = []
    for i in range(len(pos_tags)):
        doc[i].pos_ = pos_tags[i]

No functional changes - maintains 100% backward compatibility.

Original prompt

Improve python code

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