jQuery UI Themes Packer: Create it

ThemesPacker is used to build jquery-ui-themes-[VERSION].zip by the
`build-packages` grunt task.

Ref #9743
jQuery UI Themes Packer: Extend it and export API

Extend ThemesPacker with includeJs option, so it builds the CDN package.

Export ThemesPacker API, so release script is able to use it.

Fixes #9743
Closes gh-202