Refactor 'CMarkup::Format' and 'CMarkup::x_TextToDoc'

[OliverGlandberger]

push2312 observed in THIS issue that building with CMake MinGW Makefiles produces errors (Microsoft-specific strcpy_s and _vsnprintf_s functions not being "declared in this scope").

cktii observed in THIS issue that CMarkup::x_TextToDoc contains a vulnerability.

This PR refactors CMarkup::Format to resolve those issues and includes minor improvements:

• Replaced _vscprintf with vsnprintf for buffer size calculation on non-MSVC platforms, enhancing cross-platform compatibility
• Used va_copy to safely duplicate the va_list when determining buffer size, preventing potential undefined behavior from double evaluation
• Update x_TextToDoc to use std::string

[OliverGlandberger]

An additional issue was observed during testing; more investigation required before proceeding.