Separate default_langcode from system site settings

Problem/Motivation

Because of the way domain config and domain config ui works, the default langcode in system site settings form is overridden every time it is saved with the default non-overridden setting. That is because there is no form ui field on the system site settings form for default langcode. It is managed in the languages admin overview.

Steps to reproduce

Install domain config ui and domain language negotiation. Add multiple domains. Set the default langcode in the language admin overview (/admin/config/regional/language). Change the default langcode to a different language in the domain language module. Export the config.

Than go to the system site settings form and just save it. The default langcode in the overridden domain config for system site settings will be reverted to the default langcode in system.site.yml.

Proposed resolution

Take out the default language from the system.site.yml and move it to the domain.config.[DOMAIN_ID].language.negotiation.yml.
Change the language negotiator to first check this value before falling back on the (overridden) system.site.yml.