Update to new starterkit.yml file

Problem/Motivation

Recently with Drupal 10.3 a new starterkit.yml file has been introduced as mentioned here. The generate-theme command will now look for a theme_name.starterkit.yml file within the theme's root, this replaces the starterkit: true within the theme’s info.yml file.

The theme generator command will loop through all files and filenames and do a string replace of the machine_name of the starterkit theme with the machine_name of the new theme which may mean we can reduce some of the work being leveraged by our custom generator.

Proposed resolution

Create starterkit.yml file and remove key in info.yml file
Review and test generator for new options provided by the yml file.