language_default converted to CMI

This change is part of the overall effort to convert all variables to state and/or config systems.

In Drupal 7, we used variable_get() and variable_set() to store variables such as language_default to the variables table.

In Drupal 8, we are using the configuration management architecture instead. For more background, see upgrading Drupal 7 Variables to Drupal 8 configuration.

Changes introduced

This change introduces the LanguageDefault container service.

The service declaration for LanguageDefault is in core.services.yml.

Config objects now have a new property Config::getOriginal() which returns a Config object's original data values.

A new DeleteDefaultLanguageException was introduced to ensure that language_delete() does not delete the default language.