fix(release): publish Open VSX-compatible package

[ahnafnafee]

Summary

• build a dedicated Open VSX package that omits mxschmitt.postscript, which is unavailable in that registry
• keep the canonical manifest and standard VSIX unchanged so Visual Studio Marketplace users still receive the language dependency automatically
• validate both packaged manifests and add regression tests for dependency filtering

Root cause

The failed release job completed its tests, packaging, artifact upload, and GitHub Release steps, then failed during Open VSX publication with Cannot resolve dependency: mxschmitt.postscript. Open VSX resolves extension dependencies against its own registry, where that extension is not published.

The ovsx --no-dependencies option does not bypass extension dependency validation when publishing a prebuilt VSIX, so this PR creates a registry-specific artifact instead.

Verification

• yarn lint
• yarn npm audit --all --recursive
• yarn test (2 tooling tests, 9 unit tests, and 7 VS Code integration tests)
• go run github.com/rhysd/actionlint/cmd/actionlint@latest .github/workflows/release.yml
• packaged-artifact checks confirming the standard VSIX retains the dependency and the Open VSX package omits it from both manifests

Release behavior

The extension remains at version 0.6.1. Merging this PR triggers the release workflow again; the normal VSIX remains the GitHub Release and Visual Studio Marketplace artifact, while Open VSX receives the compatible package.