[JS] feat: standalone express router

[debkanchan]

create a new function called createFlowsServer that creates a flow server but does not automatically start it. Instead it returns an express server so that it could be used alongside non flow routes

[debkanchan]

This pr is based on top of this pr. Merge that pr before this pr to avoid unwanted changes.

[pavelgj]

I'm going to close this PR. #196 was merged, but we discussed this in chat and I think this can easily be achieved by defining a custom express server.