Add 'environment' package

This commit adds a new environment package that manages all calls to the
underlying operating system.

pkg/environment/native_exec.go

@@ -0,0 +1,44 @@
+package environment
+
+import (
+	"os/exec"
+
+	"github.com/tkw1536/goprogram/stream"
+)
+
+// Exec executes a system command with the specified input/output streams, working directory, and arguments.
+//
+// If the command executes, it's exit code will be returned.
+// If the command can not be executed, returns [ExecCommandError].
+func (Native) Exec(io stream.IOStream, workdir string, exe string, argv ...string) int {
+	// setup the command
+	cmd := exec.Command(exe, argv...)
+	cmd.Dir = workdir
+	cmd.Stdin = io.Stdin
+	cmd.Stdout = io.Stdout
+	cmd.Stderr = io.Stderr
+
+	// run it
+	err := cmd.Run()
+
+	// non-zero exit
+	if err, ok := err.(*exec.ExitError); ok {
+		return err.ExitCode()
+	}
+
+	// unknown error
+	if err != nil {
+		return ExecCommandError
+	}
+
+	// everything is fine!
+	return 0
+}
+
+func (n Native) LookPathAbs(file string) (string, error) {
+	path, err := exec.LookPath(file)
+	if err != nil {
+		return "", err
+	}
+	return n.Abs(path)
+}