Make sure that all examples from extension-examples work

[krassowski]

Problem

As discussed in #25 examples are great. #86 adds a way to view examples, however only a few of them actually execute in the simplified plugin-playground environment.

Proposed Solution

• Support the required loading mechanisms to ensure all examples work; importantly on lite:
  • schema population
  • relative imports
• Test that examples compile on CI. Testing their functionality is probably a non-goal, but at least we should make sure they can be executed
• Add missing known modules notably @jupyter-notebook/application

We can make it a meta-issue and track individual tasks separately.

Additional context

Some quick testing of examples:

• cell-toolbar - executes, adds commands - works
• clap-button-message - works
• codemirror-extension - works
• commands - works but we should explore a way to show console results in UI by adding - tracked in Expose logs from JS console in UI #99
• completer - works
• contentheader - works
• context-menu - works
• custom-log-console - works
• datagrid - works
• documents - works
• hello-world - works
• kernel-messaging
• kernel-output
• launcher
• log-messages
• main-menu
• metadata-form
• mimerenderer
• notifications
• react-widget
• server-extension
• settings
• shout-button-message
• signals
• state
• toolbar-button
• toparea-text-widget
• widgets

[krassowski]

I updated the description above to reflect that relative imports and schema does work (well I implemented it in #28) when running on Binder but not on JupyterLite. Thus, most examples do work on binder, but the contents API differences make them fail as of now on Lite. We need to improve how we setup examples, or how we fetch contents.