Add a reference to the FilePersister to moduleVU

The file persister will either be a local one or a remote one. The
decision will be made when the moduleVU is created, and one will be
initialised and set in the moduleVU instance.

browser/modulevu.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/grafana/xk6-browser/common"
 	"github.com/grafana/xk6-browser/k6ext"
+	"github.com/grafana/xk6-browser/storage"
 
 	k6modules "go.k6.io/k6/js/modules"
 )
@@ -20,6 +21,8 @@ type moduleVU struct {
 	*browserRegistry
 
 	*taskQueueRegistry
+
+	FilePersister storage.FilePersister
 }
 
 // browser returns the VU browser instance for the current iteration.