Supply the publisher credential for VS Code Marketplace releases

The VS Code extension release builds and passes its tests, then fails at Marketplace publication because this repository has no VSCE_PAT publisher credential. The package release is successful; this is a separate workflow.

Observed in the 0.2.719 extension release. The same missing credential was recorded after the distinct extension-ID correction in #2020.

Acceptance:

• Add the existing authorized Stacks Marketplace publisher credential as the repository's VSCE_PAT Actions secret through the normal owner-controlled secret setup.
• Rerun the existing extension workflow without bypassing publication or weakening its checks.
• Verify the published Stacks.vscode-stx version and retain the successful workflow result.

Do not expose credential values in issues, logs or artifacts. Source changes, successful package releases and the DevTools ZIP do not establish a successful Marketplace publication.

The 0.2.720 Marketplace run still fails because VSCE_PAT is missing in this repository: failed extension publication.

The compiler/runtime source CI and normal STX releaser passed. Their original package archives are verified and adopted through Stacks 0.75.329. This issue remains open for the publisher credential; the Marketplace failure has not been bypassed or described as successful.

The Marketplace credential is now configured and verified for the Stacks publisher. The rerun reached the actual publish request, so the missing-secret failure is resolved.

Marketplace then rejected stx Language Support because its display name is too similar to existing extensions. The source manifest and installation documentation now use the distinct name stx Templates by Stacks; the publisher and extension ID remain Stacks.vscode-stx. Publication remains unverified until the normal release succeeds with that correction. Failed publish request.

stx Templates by Stacks 0.2.722 is published. The Marketplace workflow succeeded after credential configuration and the display name correction.

Verified the actual Marketplace download: its manifest identifies Stacks.vscode-stx@0.2.722, all 25 ZIP members pass CRC checks, and the packaged TypeScript server plugin loads under Node as a CommonJS factory.