JS: Move the binderhub API client to its own npm package

[yuvipanda]

Many projects want / need to connect to the BinderHub API:

• thebe
• prototype UI for connecting to a private binderhub from profile_list

These all need to recreate a JS client interface to the BinderHub API.

This PR splits out the existing client API we have in the BinderHub JS codebase into its own package, and refers to that from the existing codebase. This has several advantages:

• Multiple projects can re-use this client code easily, without having to copy-paste.
• Separation of concerns allows us to eventually do wonderful things like add unit tests for the JS code.
• By using the npm workspaces feature, extra complexity overhead is very minimal. Note how small this PR actually is.

As added incentive, I've added doc strings for the methods in BinderImage. I will work on refactoring the JS code in there as well, but as a separate PR.

[yuvipanda]

https://github.com/jupyterhub/binderhub/issues/1373 is related, and in some ways I guess it's me picking up the work I was doing there :)

[yuvipanda]

AAAAND published! https://www.npmjs.com/package/@jupyterhub/binderhub-client