Issue #2276219 by Wim Leers: Asset libraries should declare their license.

core/core.libraries.yml

...et/Exception/LibraryDefinitionMissingLicenseException.php

core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php

...ts/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php

...ests/Core/Asset/library_test_files/licenses.libraries.yml

...ary_test_files/licenses_missing_information.libraries.yml