Jetpack DNA: Create a build process for package assets

#### Is your feature request related to a problem? Please describe.
In some of the packages, we have JS and/or CSS assets. They'll need a build process for development and production.

#### Describe the solution you'd like
A package should be able to define its assets and run different build processes for them:
- Minified and uglified for production (with/without source maps)
- Original with a watch mode for development

#### Describe alternatives you've considered
We can either build a new build process for packages, or use the existing build process and allow it to run for packages.

#### Additional context
Part of the Jetpack DNA project - p1HpG7-70O-p2

Start by mapping the packages that contain JavaScript or CSS assets and reviewing the repository's existing build process. Determine how package asset definitions should connect to development watch mode and production builds with optional source maps. Done means the approach is documented and supports both original development assets and minified production assets.

Written by the indexing model from the issue text.