Added internalPackages metadata entry to package.json

- our internal tooling needs to know when to add LICENSE files
- unfortunately, setting a package to private gives it a LICENSE file of
  a closed source app, which is not the case here
- my MVP solution is just to add a key here so I can detect it in Slimer

package.json

@@ -11,6 +11,7 @@
   ],
   "monorepo": {
     "public": false,
+    "internalPackages": true,
     "repo": "https://github.com/TryGhost/Ghost",
     "scope": "@tryghost"
   },