Option to opt-out for public bundles

[daklik]

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.

[boutell]

While a public UI bundle with no actual code in it doesn't take long, zero would be a more appropriate amount of time for sure.

[daklik]

In fact, the purpose would be also for dev environment, where APOS_DEV is not set, to build nothing then on a server restart in this case, not even the src build.