[VS Code] Allow disabling automated pre-release with a CI flag

Problem to solve

In some cases it might be useful to toggle the automated pre-release - thinking of the case where there's a known bug and there isn't an easy fix/rollback. Or in the case of pipeline issues that prevent a merge from going through.

To disable it today, it appears we'd need to successfully update the config in the codebase, which could potentially be blocked in certain scenarios (e.g. pipeline issues), requiring a manual 'immediate merge'.

The CI var would allow us to more easily prevent known bugs reaching any users (including internal users) who may have enabled Pre-Releases in the extension marketplace.

Proposed solution

Suggestion: have the pre-release script check a new ENABLE_PRE_RELEASE CI variable that can be toggled by any project maintainers.