Stl support: Add support for class cm::filesystem::path

This MR provides an implementation of std::filesystem::path for compilers not supporting it natively (This is a feature introduced in C++17).

The class std::filesystem::path provides a portable way for handle paths operations independantly of the OS filesystem: Only syntactic aspects of paths are handled: the pathname may represent a non-existing path or even one that is not allowed to exist on the current file system or OS.

This class can be a strong foundation to solve #16228 or #19568 (closed) as well as the proposition to introduce cmake_path command.

Topic-rename: stl-support-filesystem-path