Add a theme configurator so recipes can generate themes

I think the YAML configuration would be something like this where a theme might want to make use of values provided by the InputConfigurator.

input:
  theme_name:
    data_type: string
    constraints:
      NotBlank: []
    prompt:
      method: ask
   default:
     source: value
     value: 'my_theme'

config:

theme:
  -
    name: my_theme
    #name: '${theme_name}'
    description: 'A description of your theme.'
    enable: true
    starterkit: 'starterkit_theme'
    path: 'themes/custom'

Much of theme generation is in GenerateTheme command, which may be better someplace else in core.

Love this idea!

From @mglaman, potential things that may need to be done as part of this:

• Core should expose a way to do batch API over CLI. Basically what Drush has for it as a proper API
• Extract command to a service that uses batch, command basically runs that batch
• Allows recipe runner to use that batch for the theme generation

I think this should be a series of issues, but going for a "big bang" fix just to prove its possible.

I started working on this and realized that it will cause a whole bunch of internal things needing to be exposed as public static methods and also cause us to lose some of the verbosity output when generating a command.

There is also another issue if the spun out code uses TranslatableMarkup because it requires a container and the GenerateTheme command was made specifically to work without an installed Drupal site.

In an interim I wonder if we can just add `theme` as proposed and in \Drupal\Core\Recipe\RecipeRunner::toBatchOperationsRecipe we had a matching method which runs the command over CLI. The biggest problem would be timeouts since we cannot batch it into smaller operations. Is that acceptable?