Option to opt-out for public bundles

## The problem to solve

With the possibility we have now to use an external front integration like `apostrophe-astro` we should have an option to tell Apostrophe to build only admin UI (aka apos) bundle, as the public front end is no more served by Apostrophe.

## Proposed solution

Add an option to `@apostrophecms/asset` in order to tell the module to build only the admin UI (aka. apos) bundle.

Start by inspecting @apostrophecms/asset's bundle-building entry points and existing tests, if present. Done means an option can produce only the admin (“apos”) bundle while omitting the public bundle; verify that behavior with the module's test or build workflow.

Written by the indexing model from the issue text.