Improve jbrowse npm usage workflow

[cmdcolin]

Trying to

import Browser from '@gmod/jbrowse'

Does not necessarily work, the main export is not correct it refers to browser/main.js (electron)

Then there are a couple other crutches such as requiring a jbrowse_conf.json and jbrowse.conf, even if you want to programmatically supply the full config

https://github.com/cmdcolin/jbrowse_in_react_app demonstrates a couple things but it was requested to allow these two above fixes