fix(compat): Add drop-in std::filesystem header for older systems

[stv0g]

Needed for some older compilers which do not include std::filesystem by default.

[n-eiling]

Looks good.
Looks in the CI as if ghc_filesystem is the default? Shouldn't we prefer std::filesystem?
Also, do you think it's a good idea to include the third party code as a copy in this repo? I always prefer to download it from the original repository. This reduces license issues and nobody can get the idea of modifying the file. Also updating the dependency should be cleaner.

[stv0g]

Looks in the CI as if ghc_filesystem is the default? Shouldn't we prefer std::filesystem?

Dang, I've implement a CMake check to see if the library is available.. However, that does not seem to work.. I will check.

Also, do you think it's a good idea to include the third party code as a copy in this repo?

I agree. But I am not sure how we want to implement this? Git submodules? CMake's ExternalProject? Or via out deps.sh shell script?

[stv0g]

I've removed the external header file. Instead we now rely on CMake's find_package() for finding the header-only library (ghc::filesystem has good CMake support for it to work flawlessly).

[stv0g]

And the deps.sh script has been adjusted to also install the library.

[n-eiling]

Looks great :)