Add the possibility to collect end years

Thank you @cslevy and @tim-diels for the patch.

I fixed "formatter settings are not getting saved for some reason". The problem was in '#name' => 'display-end-year'
Also changed "Merge same years" options summary to output as boolean.

For some reason the patch from #7 it's not applying for me when installing with composer. I recreated the patch, which applies

Sorry previously updated the wrong patch.

The patch #9 did not work for me.
I created a new one based on #7, please review it.

I also added a validation to ensure that the end date is not earlier than the start date.

hi
Reading this and just wanted t share/discuss/have-an-idea about info .

Does this functionality is exactly the one provided by this module https://www.drupal.org/project/start_end_year_only →
Thanks