Declare `data` in jquery.endless-scroll

This ensures that `data` is defined in the vendored
jquery.endless-scroll. This fixes a bug where endless scroll would not
work in development when using Vite instead of Webpack.

vendor/assets/javascripts/jquery.endless-scroll.js

@@ -62,6 +62,7 @@
     };
 
     var options       = $.extend({}, defaults, options),
+        data          = false,
         firing        = true,
         fired         = false,
         fireSequence  = 0,