Allow starterkit themes to control how the theme is generated

In #3206219: Allow configuring which theme is used as a starterkit theme, we found areas of starterkit themes that may not be covered. This was done by allowing Olivero to be used as a starterkit in manual testing.

Specifically this comment: #3206219-11: Allow configuring which theme is used as a starterkit theme

Some of these may be gaps in the default centralized process and some may need to be handled by the themes themselves.

ahh! Posted summary change in the wrong issue.

I imagine a file that can be autoloaded/discovered like a module's ServiceProvider class. This has to implement an interface which contains various integration hooks. Maybe just one big one. It doesn't get copied over, though, to the new theme.

This is the escape hatch for weird bugs until they get fixed in Drupal core.

This is the code I'm referring to

    // Load each module's serviceProvider class.
    foreach ($module_filenames as $module => $filename) {
      $camelized = ContainerBuilder::camelize($module);
      $name = "{$camelized}ServiceProvider";
      $class = "Drupal\\{$module}\\{$name}";
      if (class_exists($class)) {
        $this->serviceProviderClasses['app'][$module] = $class;
      }

Going to take a stab at this!

I don't remember our exact notes from our Slack discussions/calls. But I think the idea was:

* Themes provide a specific class to be invoked, which doesn't get copied over
* The class is invokable (__invoke) so we don't have to mess with methods names and bikeshedding, open-ended API
* The class __invoke should receive the destination path so it can make changes

Tossing up initial proof of concept. Words will need to be tossed around and decided upon.

Changing the component to "Starterkit theme"

This looks great.