VSCE: Add VSCE nightly build and automated release to buildkite

Created by: abeatrix

Task list for implementing nightly build and automated release to our CI.

• Merge https://github.com/sourcegraph/sourcegraph/pull/34537 for #26437 (closed) to Nightly Build branch
• Merge https://github.com/sourcegraph/sourcegraph/pull/34522 for #34205 (closed) to main
• Create branch for VSCE release: vsce/release
• Get a Personal Access Token for publishing extensions to VS Code Marketplace
• Add Token to onepassword under VS Code Marketplace Token
• Add VS Code Marketplace Personal Access Token to GCP Secret Manager (need devX's help: thread) https://github.com/sourcegraph/infrastructure/pull/3276
• Create a new build for the automated release #34205 (closed) on buildkite
• Create a new scheduled build for #26437 (closed) on buildkite

Scheduled build created in pipeline:

Resources:

• Current release workflow for main app
• Sourcegraph Release Issue Template
• VS Code Docs on automating release
• Workflow example