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

packages/frontend-dev-utils/README.md

@@ -0,0 +1,19 @@
+# @backstage/frontend-dev-utils
+
+Utilities for developing Backstage frontend plugins using the new frontend system.
+
+This package provides a minimal helper for wiring up a development app from a `dev/` entry point, making it easy to run and test frontend plugins in isolation.
+
+## Installation
+
+Install the package via Yarn:
+
+```sh
+cd plugins/<plugin> # if within a monorepo
+yarn add -D @backstage/frontend-dev-utils
+```
+
+## Documentation
+
+- [Backstage Readme](https://github.com/backstage/backstage/blob/master/README.md)
+- [Backstage Documentation](https://backstage.io/docs)