Add @backstage/frontend-dev-utils package

Adds a new `@backstage/frontend-dev-utils` package for the new frontend
system. It provides a minimal `createDevApp` helper for wiring up a
development app from a `dev/` entry point.

The app-visualizer plugin is updated to use this new package as the
initial testing ground.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

plugins/app-visualizer/package.json

@@ -43,7 +43,7 @@
   },
   "devDependencies": {
     "@backstage/cli": "workspace:^",
-    "@backstage/frontend-defaults": "workspace:^",
+    "@backstage/frontend-dev-utils": "workspace:^",
     "@types/react": "^18.0.0",
     "react": "^18.0.2",
     "react-dom": "^18.0.2",