How do I get the route name from the template?
There is a build-in variable routeName, so you can just use it in your templates {{ routeName }}
Is @import supported in .scss/.sass files?
Yes, just provide a path related to "app.assets" directory. Ex: @import './styles/variables';
How to add custom functions or filters for twig?
Just add an additional .js file in your project and provide the path in web-boost config file.
See details here