Use domains and slugs at the same time

[ivanvermeyen]

Enable the combined use of localized domains and slugs.

Fictional use case:

Locale	URL	Description
en	https://example.be/en	English
nl	https://example.be/nl	Dutch
nl_BE	https://example.be/nl	Belgium, Dutch
fr_BE	https://example.be/fr	Belgium, French
nl_NL	https://example.nl/	Netherlands, Dutch (no slug)
fr_FR	https://example.fr/	France, French (no slug)

Possible configuration (related to suggested configuration in #20):

'supported-locales' => ['en', 'nl', 'nl_BE', 'fr_BE', 'nl_NL', 'fr_FR'],

'domains' => [
    'en' => 'example.be',
    'nl' => 'example.be',
    'nl_BE' => 'example.be',
    'fr_BE' => 'example.be',
    'nl_NL' => 'example.nl',
    'fr_FR' => 'example.fr',
],

'slugs' => [
    'en' => 'en',
    'nl' => 'nl',
    'nl_BE' => 'nl',
    'fr_BE' => 'fr',
    'nl_NL' => null, // no slug
    'fr_FR' => null, // no slug
],

This is just a first idea of how this might look. Will need to work out if any misconfigurations can happen (we need to be able to detect the locale based on the URL) and how to handle that.