Zeitwerk support

[yui-har]

When I tried to run this plugin with Redmine's trunk branch(r21444), the execution of bundle exec rake db:create failed.

rake aborted!
LoadError: cannot load such file -- redmine_extended_reminder/hooks
/tmp/redmine/vendor/bundle/ruby/3.1.0/gems/zeitwerk-2.5.4/lib/zeitwerk/kernel.rb:35:in `require'
/tmp/redmine/vendor/bundle/ruby/3.1.0/gems/zeitwerk-2.5.4/lib/zeitwerk/kernel.rb:35:in `require'
/tmp/redmine/vendor/bundle/ruby/3.1.0/gems/activesupport-6.1.4.6/lib/active_support/dependencies/zeitwerk_integration.rb:51:in `require_dependency'
/tmp/redmine/plugins/redmine_extended_reminder/init.rb:2:in `<top (required)>'

I fixed to work with the trunk and the following versions of Redmine.

• trunk(r21444)
• 4.2-stable(r21431)
• 4.1-stable(r21432)
• 4.0-stable(r20977)
• 3.4-stable(r19398)