Support opening and saving .docm (macro-enabled) Word files without stripping macros.

Current behavior

Opening a .docm and saving it strips the VBA project because python-docx doesn't preserve the vbaProject.bin part.

API Design

• Document(path) — auto-detect .docm and preserve macro parts
• document.save('output.docm') — save with macros preserved
• document.has_macros — bool indicating if VBA project is present

Implementation

• Preserve the vbaProject.bin binary part and its relationship during round-trip
• Use correct content type for .docm: application/vnd.ms-word.document.macroEnabled.12
• Don't modify or parse the VBA part — just preserve it

Upstream: python-openxml#284, python-openxml#716 (29+16 = 45 comments)