add mousewheel functionality to lightbox

For a start you have a syntax error, which the error console would confirm.

lightbox.trigger('.fslightbox-slide-btn-container-next"'); //<-- "

Secondly, you seem to be making considerable assumptions on what trigger() does and how it works, believing that you pass it a selector and it triggers a click event. That’s not how it works. I think you mean:

lightbox.find('.fslightbox-slide-btn-container-next').click();