Add static preloads notebook contribution

[mjbvz]

For #160965

This tracks a new contribution point that lets extensions contribute static preload scripts to a notebook of a specific type. Notebooks can use this to load global scripts before any notebook renderers have been loaded

[mjbvz]

A few notes from my meeting with @DonJayamanne about this setting

• Jupyter is already using it to load scripts such as require.js

• Ideally this new contribution point would also replace the NotebookRendererScript API proposal

  • Jupyter currently uses this to load two things: a script that is only used for testing the Jupyter extension, and ipywidgets
  • ipywidgets should be easy to migrate. For the test script, we could potentially just allow a preload to be marked as optional so it doesn't matter if it doesn't exist (or jupyter could ship an empty test script file for production)

[DonJayamanne]

@mjbvz I have updated the jupyter extension to use the static contribution for ipywidgets.
The only usage of this old API is here
Jupyter currently uses this to load two things: a script that is only used for testing

I will update our test code to inject the static contributions for the test scripts as well, this way things work when testing.
& also update our CI pipeline to ensure this never gets checked into source,

this way, the static contribution for the tests script is only added for testing/development or on CI, but will never get shipped
I.e. Jupyter extension should not have the need for this dynamic contribution in the controller.

[mjbvz]

We've going to leave this proposed until we have other use cases for it beyond jupyter