Adds configurable config-split enabling as part of installation

This release requires config_split:^2.0

Recommended that you add a protected function getConfigSplits() to your robofile when installing this update.

Up to now, we always assumed you had a local config file called "local". If that's true of your site, and that's the only local config split you have, you don't need to do anything!

If your local config split has a different machine name, like "personal", you'd add that to your Robofile like this:

protected function getConfigSplits() {
    return ['personal'];
  }

You can also isolate out the config_suite module if you find your site getting unintended changes to the core.extension.yml file frequently, by making it its own config split and adding that to the list of config items to update.

protected function getConfigSplits() {
    return ['local', 'config_suite_only'];
  }