Add 'environment' package

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

pkg/environment/environment.go

@@ -0,0 +1,58 @@
+package environment
+
+import (
+	"io"
+	"io/fs"
+	"net"
+	"time"
+
+	"github.com/tkw1536/goprogram/stream"
+)
+
+// Environment represents an environment that a program can run it.
+// It mostly mimics the interfaces of the [os] package.
+type Environment interface {
+	isEnv()
+
+	GetEnv(name string) string
+
+	Stat(path string) (fs.FileInfo, error)
+	Lstat(path string) (fs.FileInfo, error)
+
+	Readlink(path string) (string, error)
+	Symlink(oldname, newname string) error
+
+	ReadDir(name string) ([]fs.DirEntry, error)
+
+	Open(path string) (fs.File, error)
+	Chtimes(name string, atime time.Time, mtime time.Time) error
+	SameFile(f1, f2 fs.FileInfo) bool
+
+	Create(path string, mode fs.FileMode) (WritableFile, error)
+
+	Mkdir(path string, mode fs.FileMode) error
+	MkdirAll(path string, mode fs.FileMode) error
+
+	Remove(path string) error
+	RemoveAll(path string) error
+
+	WalkDir(root string, fn fs.WalkDirFunc) error
+
+	Abs(path string) (string, error)
+
+	Listen(network, address string) (net.Listener, error)
+	Dial(network, address string) (net.Conn, error)
+
+	Executable() (string, error)
+	Exec(io stream.IOStream, workdir string, exe string, argv ...string) int
+	LookPathAbs(name string) (string, error)
+}
+
+type WritableFile interface {
+	fs.File
+	io.Writer
+}
+
+func init() {
+	var _ Environment = Native{}
+}