Adopt to pre-release extension support

[isidorn]

Refs: microsoft/vscode#15756

VS Code now supports pre-release extensions. Docs that should help: https://code.visualstudio.com/api/working-with-extensions/publishing-extension#prerelease-extensions

I know Python uses the setting to move users to an insiders version.
This is a bit of a special case since you do not have a separate extension so we do not support migration here. However we suggest that Python does the following for users that have this settings set:

• Show a modal dialog or notification to user where she can confirm the transition
• Once user confirms, execute the command workbench.extensions.installExtension and pass { installPreReleaseVersion: true }
• Deprecate the setting

Let me know what you think or if I can help somehow.

fyi @brettcannon @sandy081

[brettcannon]

Was going to track this in favour of #17196, but I'm going to reverse it and close it in favour of this one instead (i.e. this is already on our roadmap and planned for 😃).

[isidorn]

@brettcannon sorry, did not see that one! Thanks for merging them and thanks for working on this.

[luabud]

I think we will also need to update what we define as an insider user for experimentation (ExP) purposes

[tanhakabir]

Not sure if the pre-release is out yet but I'm still seeing these notifications on any workspace that I have .py or .ipynb files in:

[karrtikr]

It's not out yet so this is expected.