Add select2 dependency in libraries.yml file

Tested with Drupal 10.1.2 with field formatter and better exposed filters in a view and both work well. Without this patch I was only seeing select2 working, never select2 multicheck.

This fixes the problem for me whereby on initial page load the basic select2 widget was used on a field, it didn't switch to select2_multicheck until after I first checked one of the items. RTBC!