Split jupyverse-api into a package for each plugin

[davidbrochart]

Problem

Currently, Jupyverse's public API is in a single package jupyverse-api, that includes every plugin's public API.
This is not ideal because every time a plugin's public API changes, the package has to be released.

Proposed Solution

JupyterLab's packages follow a different scheme, for instance:

• the notebook package consists of the public API.
• the notebook-extension package uses the notebook public API to implement a notebook plugin.

I think Jupyverse should follow the same approach, and extract out each plugin's public API in jupyverse-api into its own package.