Add abbreviation name for generator

adding support for an abbreviation of a newly created theme within a generator, and this abbreviation will be used to update/replace CSS property prefixes. To achieve this, we typically need to update a configuration file in the project.

As per my knowledge we have to add abbreviation in configuration file within the theme that handles settings related to the CSS generation or prefixing. In prototype theme this could be a .info.yml.
So we add abbreviation name for generator in .info.yml file.

I've updated the generator to allow users to pass an abbreviation name for css variables during initial theme setup. During testing this allows users to set a folder name and theme name independant of the css variable names. It is an optional element that can be passed during generation.

This has been merged into the 5.x branch, we will include it in the next tagged release.