feat(convert): integrate md4c for bidirectional markdown sync (Phase 1)

[mandeepsingh2007]

Description

This Pull Request introduces the foundational Phase 1 integration for the Advanced Markdown-to-TMU Bidirectional Sync feature, utilizing the highly performant md4c library.

As discussed, this initial PR focuses strictly on the build system integration and basic C++ plumbing to keep the review process focused and incremental.

Changes Made

• Build System: Added md4c (C Markdown parser with GFM support) as a 3rd-party dependency in xmake.lua.
• C++ Bindings: Created placeholder files markdown.hpp and markdown.cpp inside src/Data/Convert/Markdown/.
• Input Pipeline Workflow: Updated texmacs_input_rep::markdown_flush in src/Data/Convert/Generic/input.cpp to call the native C++ markdown_to_tree adapter, successfully bypassing the legacy Scheme fallback layer.
• Compatibility: Added a small patch to 3rdparty/s7/s7.h to fix a MSVC bool macro redefinition error affecting modern compilers during C builds.

Testing

• Locally compiled on Windows using xmake.
• Since the C parser is just a placeholder returning the raw text string wrapped in a document node, logic-heavy TMU mapping will be introduced in the upcoming (Phase 2) PR.

[mandeepsingh2007]

@JackYansongLi please review this