Add EML email conversion support

[Lubrsy706]

Summary

• add a built-in RFC 822 / .eml converter using Python's standard email parser
• emit email metadata, preferred plain-text content, HTML fallback conversion, and attachment names
• add an EML fixture to the existing vector conversion tests

Fixes #89.

Verification

• .venv/bin/python -m pytest packages/markitdown/tests/test_module_vectors.py::test_guess_stream_info packages/markitdown/tests/test_module_vectors.py::test_convert_local packages/markitdown/tests/test_module_vectors.py::test_convert_stream_with_hints packages/markitdown/tests/test_module_vectors.py::test_convert_stream_without_hints -q
• .venv/bin/python -m compileall -q packages/markitdown/src/markitdown/converters/_eml_converter.py

[Lubrsy706]

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