os: add File.Chown on the unix path

file_unix.go (darwin/linux/wasip) was missing the exported (*File).Chown method
that file_other.go (baremetal/wasm) already has, so callers requiring the full
os.File surface — e.g. github.com/pkg/sftp — failed to compile for the
linux/amd64 TinyGo target. Delegates to the package-level Chown, mirroring
(*File).Truncate.

src/os/file_unix.go

@@ -163,6 +163,17 @@ func (f *File) Truncate(size int64) (err error) {
 	return Truncate(f.name, size)
 }
 
+// Chown changes the numeric uid and gid of the named file.
+// A uid or gid of -1 means to not change that value.
+// If there is an error, it will be of type *PathError.
+func (f *File) Chown(uid, gid int) error {
+	if f.handle == nil {
+		return ErrClosed
+	}
+
+	return Chown(f.name, uid, gid)
+}
+
 func (f *File) chmod(mode FileMode) error {
 	if f.handle == nil {
 		return ErrClosed