Add SPM support

[Coeur]

SPM is Swift Package Manager.

It's cross-platform (Darwin, Linux, Android, Windows) and it supports Swift, ObjC, C++, C, ...

It's a bit like a CMakeLists.txt, just more restrictive as it runs in a sandbox preventing running commands that could be potentially risky.

I wrote it because on macOS, double-clicking Package.swift will open an Xcode project that can be used for building, analyzing and editing the project, just like the existing par2cmdline.sln for Visual Studio.

On the caveats, due to sandboxing, it can't run ./configure, so it can't generate config.h, so I had to add the missing definitions for i8, i16, i32 and i64 when HAVE_CONFIG_H was not defined.

[BlackIkeEagle]

The main difference would be that we now don't use this swift file for building so the chance of it going out-of-date is large. With the visualstudio project file we will notice if something is missing since it is actually used for building. Is there a way for us to notice this in CI as well that this swift file is not updated correctly ?

[Coeur]

Yes, let me work on adding a CI build in the coming days.
The command would be simply swift build on a macOS environment.

[BlackIkeEagle]

ah so it could at least replace the following lines then?

https://github.com/Parchive/par2cmdline/blob/master/.github/workflows/build-check-macos.yml#L14-L24
and
https://github.com/Parchive/par2cmdline/blob/master/.github/workflows/build-release.yml#L113-L123

with swift build?

[BlackIkeEagle]

@Coeur could you find the time to answer my question? And is there additional work needed for this ?