move filemode parsing to its own package

why doesn't the std lib have this???

internal/filemode/filemode.go

@@ -0,0 +1,25 @@
+package filemode
+
+import (
+	"io/fs"
+	"strconv"
+)
+
+// Parse parses a string of 3 or 4 numbers as a *NIX filesystem permission.
+//
+// "0777" or "777" -> fs.FileMode(0777)
+//
+// "0644" or "644" -> fs.FileMode(0644)
+func Parse(in string) (fs.FileMode, error) {
+	if in == "" {
+		return fs.FileMode(0), nil
+	}
+	if len(in) == 3 {
+		in = "0" + in
+	}
+	md, e := strconv.ParseUint(in, 8, 64)
+	if e != nil {
+		return 0, e
+	}
+	return fs.FileMode(md), nil
+}