Windows Visual Studio Clang Support

Incorporated code changes that should be transparent to Linux-based builds to support building ns-3 libraries in Visual Studio using the C++ Clang tools for Windows on Visual Studio 2022.

Updates of note: CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS option added to CMakeLists.txt: exports all symbols removing the need for explicit __declspec(dllexport) calls and only requiring __declspec(dllimport) calls for certain static data members in classes.

NS3_INT64X64 default to DOUBLE for MSVC: MSVC doesn't appear to have INT128 support that cross compiles directly with Linux-based builds. Working around for now by switching to DOUBLE only for MSVC builds.

New ns-*-export.h files: Only actually needed for __declspec(dllimport) on static data members. #include <unistd.h> in config-store.cc: Not used even in Linux-based builds.

nstime.h g_markingTimes initialization: Since restriction prior to C++0x not a constraint anymore, removed current static initialization design process for direct inline initialization. DLL under baseline initialization reports an access violation attempting to access the initialized memory.