ES 模块:请进行讨论

I suspect this is a feature that many people would like, though perhaps not as many as were the case for Underscore. Personally, I think there would be value in being able to import `Backbone.Events`, `Backbone.Model`, `Backbone.Collection` and perhaps the `extend` class emulator without importing the entirety of Backbone. Modularizing Backbone has been part of stage 2 of my maintenance takeover plan (https://GitHub.com/jashkenas/backbone/issues/4244#issuecomment-993692119).