Allow Dart Sass transformations to be cached on disk

This enbles people to build their sites/themes on servers where the Dart Sass binary is not available.

resources/transform.go

@@ -51,9 +51,11 @@ var (
 // These are transformations that need special support in Hugo that may not
 // be available when building the theme/site so we write the transformation
 // result to disk and reuse if needed for these,
+// TODO(bep) it's a little fragile having these constants redefined here.
 var transformationsToCacheOnDisk = map[string]bool{
-	"postcss": true,
-	"tocss":   true,
+	"postcss":    true,
+	"tocss":      true,
+	"tocss-dart": true,
 }
 
 func newResourceAdapter(spec *Spec, lazyPublish bool, target transformableResource) *resourceAdapter {