copied from csells/go_router#374

Hi, I'm in middle of developing Web page in Flutter. The deeper I go the main file called main.dart.js is getting bigger. So then I've started searching how to reduce the file size.

I came across to that article and found out about deferred libraries. That sample project from article is based on qlevar_router which has such handy method as Middleware which helps in implementing separate file for each page.

Since my project is based on your awesome package GoRouter, I was thinking about if are you planning to implement this or haven't thought about it yet?
Please let me know if its interesting enhancement for your package.