Allow multiple ui.frontend modules

### Expected Behaviour
The _frontendModule_ parameter should allow more than one value (for example, _general_ and _angular_), so we can create a project that supports both SPA Editor and traditional AEM.

### Actual Behaviour
The _frontendModule_ parameter allows one single value, so currently is not possible to create a project that combines SPA and non-spa.

Trace the frontendModule parameter through the archetype configuration and generated project structure, starting with where that parameter is defined and consumed. Confirm that multiple values such as general and angular can generate a project supporting both SPA Editor and traditional AEM, then validate the generated output.

Written by the indexing model from the issue text.