Add Windows-specific directory creation for gs_platform_dir_exists_default_impl

[reshen]

In msvc, turns out mkdir only takes a single argument so this code needs a minor tweak to handle that case. While I was looking at the official docs, also noticed that mkdir is deprecated in favor of _mkdir, so made that change as well.

Without this patch:

**********************************************************************
** Visual Studio 2022 Developer Command Prompt v17.14.18
** Copyright (c) 2025 Microsoft Corporation
**********************************************************************
[vcvarsall.bat] Environment initialized for: 'x86'

...

gs_project_template> proc\win\cl.bat
The system cannot find the file specified.
Microsoft (R) C/C++ Optimizing Compiler Version 19.44.35219 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

main.c
C:\Users\reshen\gamedev\learn-c\gs_project_template\third_party\include\gs\impl/gs_platform_impl.h(722): error C2197: 'int mkdir(const char *)': too many arguments for call

With this patch:

gs_project_template> proc\win\cl.bat
Microsoft (R) C/C++ Optimizing Compiler Version 19.44.35219 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

main.c
Microsoft (R) Incremental Linker Version 14.44.35219.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/debug
/out:App.exe
/SUBSYSTEM:CONSOLE
/NODEFAULTLIB:msvcrt.lib
/NODEFAULTLIB:LIBCMT
opengl32.lib
kernel32.lib
user32.lib
shell32.lib
vcruntime.lib
msvcrt.lib
gdi32.lib
Winmm.lib
Advapi32.lib
main.obj

gs_project_template> ls bin
App.exe  App.ilk  App.pdb  main.obj  vc140.pdb

p.s. Gunslinger is super cool, and your YT videos are fantastic. Appreciate you sharing your knowledge.