The diff reorders includes across multiple source files to place standard library headers before project headers, removes the `_CRT_SECURE_NO_WARNINGS` define from `wren_common.h`, and adds it as a preprocessor definition in both Debug and Release configurations of the MSVC 2013 project file.
Add wren_math.c and wren_math.h to the MSVC 2013 project files, including both the .vcxproj and .vcxproj.filters. Also fix the indentation of the WREN_COMPUTED_GOTO comment in wren_common.h and correct a conditional guard bug in wren_vm.c where wrenLoadMathLibrary was guarded by WREN_USE_LIB_IO instead of WREN_USE_LIB_MATH.
Add MSVC 2013 project scaffolding under project/msvc2013/ including wren.sln solution file, wren.vcxproj with Debug/Release Win32 configurations using v120 toolset, and wren.vcxproj.filters organizing source and header files from ../src/ into Visual Studio filter categories.
- Relocate wren.xcodeproj into project/xcode/ subdirectory
- Remove build_xcode/ from .gitignore as it is no longer at root level
- Update all internal PBXBuildFile references to use new file paths
