Fix Windows build for cl.exe and clang targeting MSVC

[zzyxyz666]

Make the build and tests work on Windows with both MSVC cl.exe and
clang targeting x86_64-pc-windows-msvc, in both C and C++ modes.

• shared.h: for cl.exe add /utf-8 to the C (/TC) and C++
  (/std:c++20 /TP) flag sets so UTF-8 sources compile correctly on
  Windows. For clang targeting MSVC use clang-style flags (-std=c99 /
  -x c++, -Wall, -Wswitch-enum, ...) instead of the MSVC-only
  flags that its driver rejects.
• nob.h: make nob_cc_output append .exe when building with clang
  targeting MSVC (-o<path>.exe) so the produced binary matches the
  ../<test>.exe run command. cl.exe already emits <path>.exe
  through /Fe, and non-Windows builds are unchanged.
• nob.h: include <consoleapi2.h> and undef the private SDK guards
  (_WINUSER_, _WINGDI_, _IMM_, _WINCON_) and
  WIN32_LEAN_AND_MEAN after the system includes so
  SetConsoleOutputCP() is declared even when compiling with
  cl.exe /TP. Also #undef UNICODE so the generic WinAPI names keep
  resolving to the ANSI variants the rest of the code uses. Undefing
  the guards is also required for downstream code that includes
  <windows.h> after nob.h to still get the winuser.h /
  wingdi.h / imm.h / wincon.h declarations (<windows.h> cannot
  be included before nob.h because of the ERROR macro clash).
• nob.c: remove the extern "C" workaround for SetConsoleOutputCP(),
  no longer needed with the header fix above.

Testing

• cl.exe /TC and cl.exe /std:c++20 /TP: all tests pass
• clang -x c and clang -x c++: all tests pass
• clang-cl /TC and clang-cl /std:c++20 /TP: all tests pass
• gcc -x c and gcc -x c++: all tests pass
• compiler versions: clang version 21.1.0 cl version 19.44.35222 gcc.exe (Rev11, Built by MSYS2 project) 15.2.0 ``

Related: #193 (an earlier, partial attempt that only covered the C flag
set and predates the C++ support split).