Break the hard dependency on the Jupyter extension

[brettcannon]

We will still install it by default, but you will at least have the option to uninstall it.

[luabud]

I was talking to @claudiaregio about this and it'd be great if we could have a notification that would pop up if users try to use some functionality that is available only in the Jupyter extension but they don't have it installed, such as the data viewer when debugging or when clicking on "Open interactive window" and "Create new Jupyter notebook" buttons from our start page.

The notification could say "This feature is available in the Jupyter extension, which isn't currently installed." and then we can add a button with "Install Jupyter extension" that would open it in the marketplace (or install it directly if we can). Users can dismiss the prompt if they don't want to.

[claudiaregio]

Yeah it would be great if we could do the above. The reason being users likely don't know what features are powered by Python vs. Jupyter and may uninstall Jupyter thinking they don't need it if they don't use notebooks. They may then think something is broken in the Python extension when no longer seeing the entry points they're used to all of a sudden.

[brettcannon]

@luabud then we are going to need a spec of exactly what functionality we have to keep a shim in for regardless of whether the Jupyter extension is installed (unless you intend that for all functionality?).

[luabud]

Great point, that's not for all functionality. Only those who are not obviously done by the Jupyter extension. They are:

• Open data viewer from the local variable window when debugging
• "Create a Jupyter notebook" button in the start page
• "Create new blank notebook" link in the start page
• "sample notebook" link in the start page.
• When clicking on the "use the interactive window to develop Python scripts" command.

We can also edit the block to change the first bullet point below to be: "- You can create cells on a Python file by typing "#%%". Make sure you have the Jupyter extension installed" (or anything else @claudiaregio thinks would be best).

@claudiaregio please let me know if you can think of other Jupyter entry points in the Python extension that would be exposed to users that don't have the Jupyter extension installed, but that would not work because of that.

[joyceerhl]

Context re: creating a shim for opening the data viewer from the local variable window: currently the 'View in Data Viewer' context menu item is contributed entirely by the Jupyter extension:
https://github.com/microsoft/vscode-jupyter/blob/844bca1ba9300230ad6b23ddd1c22752f495e31f/package.json#L1334-L1340

The item's enablement is based on a special property, __vscodeVariableMenuContext, that the Jupyter extension sets at runtime on each variable that passes through our debug adapter tracker:
https://github.com/microsoft/vscode-jupyter/blob/fb44efc4d052aa08686ff6eb77aac66c5a36cc95/src/client/datascience/jupyter/debuggerVariables.ts#L311-L318

To shim this, you would need to implement a debug adapter tracker in the Python extension which handles messages sent from the debug adapter to VS Code and sets the __vscodeVariableMenuContext property on each variable (basically replicate part of the code that we have in the Python extension):
https://github.com/microsoft/vscode-jupyter/blob/fb44efc4d052aa08686ff6eb77aac66c5a36cc95/src/client/datascience/jupyter/debuggerVariables.ts#L201-L206