add lastpass mfa dialog variant scaffolding; add yubikey variant (#6687)

apps/browser/src/_locales/en/messages.json

@@ -2674,5 +2674,8 @@
   },
   "collection": {
     "message": "Collection"
+  },
+  "lastPassYubikeyDesc": {
+    "message": "Insert the YubiKey associated with your LastPass account into your computer's USB port, then touch its button."
   }
 }