Upgrade mvdan.cc/sh to v3.0.0-beta1

internal/execext/devnull.go

@@ -0,0 +1,13 @@
+package execext
+
+import (
+	"io"
+)
+
+var _ io.ReadWriteCloser = devNull{}
+
+type devNull struct{}
+
+func (devNull) Read(p []byte) (int, error)  { return 0, io.EOF }
+func (devNull) Write(p []byte) (int, error) { return len(p), nil }
+func (devNull) Close() error                { return nil }