[Request] Lib mode for exporting a library instead of an app

[manelclos]

Allow creating a "lib" mode in which you have a "lib" directory with some index.js exporting some classes.

I'm currently doing this by modifying the entry and output config of webpack, but it is hacky. Before moving to Quasar this was handled with a different command (build:lib) in package.json and a different webpack config.

We have a base project with components we reuse in other projects.