fix: preserve underlined docx text

[he-yufeng]

Fixes #35

Summary

• map DOCX underline runs to HTML u tags by default
• keep HTML u tags in Markdown output because Markdown has no native underline syntax
• add DOCX and HTML underline regression coverage

To verify

• python -m pytest packages/markitdown/tests/test_module_misc.py -q -k "underline or docx_comments or docx_equations or input_as_strings"
• python -m py_compile packages/markitdown/src/markitdown/converters/_markdownify.py packages/markitdown/src/markitdown/converters/_docx_converter.py packages/markitdown/tests/test_module_misc.py
• git diff --check