Create a nightly/insiders extension in the Marketplace

[brettcannon]

This would replace our bespoke insiders channel download/install mechanism so we can rely on VS Code to update.

• Create a package.prod.json which contains only that which would differ from the nightly build
• Create a "nightly build" workflow (w/ uploading turned off)
• Use GitHub Environments to control whether a nightly build gets pushed to the Marketplace
• Create a custom build for blob storage that will notify people on our old insiders builds to switch to the new extension
• Remove code for blob storage solution
• Switch over to the new workflow
• Upload the custom VSIX to blob storage
• Make a stable release which would overwrite anyone on the insiders build from blob storage

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#17133

[brettcannon]

microsoft/vscode#15756 may lead to this being put on hold.

[jakebailey]

A wrench in this plan is that Pylance hard depends on the core extension, as it's not the one running the server and such; if there are two Python extensions, we'd necessarily have to drop our side of the dependency and then figure out what to do with people who discover and install the stack via the Pylance extension.

[brettcannon]

@jakebailey chances are we will probably go with the solution the VS Code core team is proposing and using the mechanism where there's just the single extension and VS Code controls which VSIX to download, but good point about how this might make it messy unless you also publish an insiders extension that relies on our insiders extension.