Fix errors when compiling with UNICODE macro, improvements to nob_win32_error_message, nob_mkdir_if_not_exists, nob_file_exists

[KillerxDBr]

• Refactored "nob_win32_error_message" to not return NULL if snprintf fail for some reason, it will always return a string now.

• Made a proper Windows API version of "nob_mkdir_if_not_exists".

• Finished "nob_file_exists", now it returns -1 in case of error. ("distinguish between "does not exists" and other errors")

• Added a ifdef to minirent static function declarations to avoid compiler warnings when using nob.h in header mode:

.\nob.h:586:13: warning: 'opendir' declared 'static' but never defined [-Wunused-function]
  586 | static DIR *opendir(const char *dirpath);
      |             ^~~~~~~

nob cant be compiled on Windows if UNICODE/_UNICODE macros are enable, so i changed all windows API calls and structs to the A version, there was some using the macro (E.g. "CopyFile" instead of "CopyFileA" or "CopyFileW") versions, that changed if those macros are defined.

E.g.

nob.h: In function 'nob_copy_file':
nob.h:720:19: error: passing argument 1 of 'CopyFileW' from incompatible pointer type [-Wincompatible-pointer-types]
  720 |     if (!CopyFile(src_path, dst_path, FALSE)) {
      |                   ^~~~~~~~
      |                   |
      |                   const char *
In file included from C:/msys64/ucrt64/include/windows.h:70,
                 from nob.h:197:
C:/msys64/ucrt64/include/winbase.h:2304:48: note: expected 'LPCWSTR' {aka 'const short unsigned int *'} but argument is of type 'const char *'
 2304 |   WINBASEAPI WINBOOL WINAPI CopyFileW (LPCWSTR lpExistingFileName, LPCWSTR lpNewFileName, WINBOOL bFailIfExists);

• Fix for "nob_read_entire_file" to be able to handle files larger than 2GB on Win32

[KillerxDBr]

I will refactor this one more time