Extract unix/spawn.cr as a separate file

[straight-shoota]

unix/process.cr contains a lot of different features and is quite big. This patch extracts the code related to spawning a new process into a separate file to improve maintainability.

[ysbaddaden]

If you want but it splits System::Process into an unexpected extra file on UNIX, and the file ain't significantly large —looking at the ~5900 lines of string.cr 😏

[straight-shoota]

It'll grow when we add alternative implementations (see #11337).

• posix_spawnp used on most targets
• clone/exec on linux for pidfd support
• fork/exec as fallback for targets which don't provide posix_spawn_file_actions_addchdir (Crystal::System::Process#spawn forks, should call posix_spawn() or posix_spawnp() instead #11337 (comment)).