Implement a base for assets

[xipasduarte]

The assets folder should be present, even without a gulp (or any other builder) present. Choose a folder name that aligns both Themes and Plugins.

Suggestion:

• Have the assets on a /public folder with a build relation as follows:

src/scripts => public/ src/styles => public/ src/images => public/images

• CSS and JS assets file names:

Frontend: plugin-slug.css|js Backend: plugin-slug-admin.css|js

[Narayon]

This should not be the common behavior because some plugins don't have assets.
A sub-generator is already on the roadmap, so I'm closing this issue.