Add Dart Sass support

But note that the Dart Sass Embedded Protocol is still in beta (beta 5), a main release scheduled for Q1 2021.

Fixes #7380
Fixes #8102

commands/server.go

@@ -526,6 +526,8 @@ func (c *commandeer) serve(s *serverCmd) error {
 		<-sigs
 	}
 
+	c.hugo().Close()
+
 	return nil
 }