Allow MyST to start its own Jupyter server for in-page execution

[choldgraf]

It's possible to point MyST to a pre-running Jupyter server for in-page execution, but this requires users to manually create the server and configure MyST to point to it.

We already have the ability to start Jupyter servers as part of the build execution process.

We should allow users to start a Jupyter server with myst start that would auto-connect as a back-end server for in-page computation. This would reduce the amount of configuration confusion, friction, and toil on behalf of users.

So for example, something like:

myst start --thebe local

or

jupyter:
  thebe: local

would spin up a server and point thebe to it automatically.

Then, when people actually launch things via thebe on the page, it'd use that server to create (or re-use) kernels.

[agoose77]

Let's add this to the execution improvements unrefined initiative. There's a larger unit of work to make execution more generalised and consistent between Thebe and build time.