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