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

.changeset/add-frontend-dev-utils.md

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+---
+'@backstage/frontend-dev-utils': minor
+---
+
+Added `@backstage/frontend-dev-utils`, a new package that provides a minimal helper for wiring up a development app for frontend plugins using the new frontend system. It exports a `createDevApp` function that handles creating and rendering a development app from a `dev/` entry point.

.changeset/app-visualizer-use-frontend-dev-utils.md

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+---
+'@backstage/plugin-app-visualizer': patch
+---
+
+Switched dev entry point to use `createDevApp` from `@backstage/frontend-dev-utils`.