Add File::BadExecutableError

[HertzDevil]

This PR maps Errno::ENOEXEC and WinError::ERROR_BAD_EXE_FORMAT to a new exception class, File::BadExecutableError, that is raised if Process.new receives a file that has execute permissions but isn't really an executable file. WinError::ERROR_BAD_EXE_FORMAT passes the file argument to LibC.FormatMessageW. Fixes #12985. (The new exception class isn't technically necessary, I just think this is a good chance to capture this error in a platform-independent way.)

Note that this doesn't happen naturally on POSIX systems, where Process.new is backed by LibC.execvp; if execvp failed because of ENOEXEC, execution of the same file is attempted again via sh as though it is a shell script. Therefore, trying to execute a Windows executable on a non-WSL Linux produces:

begin
  p Process.run("/home/quinton/crystal/crystal/crystal.exe", output: :inherit, error: :inherit)
rescue ex
  p ex
end

/home/quinton/crystal/crystal/crystal.exe: 1: MZ����@▒��: not found
/home/quinton/crystal/crystal/crystal.exe: 2: Syntax error: word unexpected (expecting ")")
Process::Status[2]

However, musl is known not to fall back to sh, in which case File::BadExecutableError could still be caught. If we want to use LibC.execv instead then we must replicate the $PATH lookup ourselves when the file name does not include a slash character.

This PR also maps WinError::ERROR_ACCESS_DENIED to File::AccessDeniedError. This happens when Process.new receives a directory on Windows.