Update changelog template (for pre-release features)

[loujaybee]

Objective

We've recently added an in-product changelog to VS Code, this issue is to update our changelog template to make sure that it highlights pre-release features to test (currently experimental ports view, SSH Gateway in VS Code Desktop)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.