🎨 Log a warning if an extension has already been loaded

If an extension has already been loaded (for instance because it’s
persisted), `addDevToolsExtension` will return nothing, which is
confusing. This adds a little `console.warn` to educate people about
what’s happening.

Closes #5854

lib/browser/chrome-extension.js

@@ -38,6 +38,8 @@ const getManifestFromPath = function (srcDirectory) {
       })
     })
     return manifest
+  } else if (manifest && manifest.name) {
+    console.warn(`Attempted to load extension "${manifest.name}", but extension was already loaded!`)
   }
 }