Add Open VSX Registry publishing workflow

[rbinksy]

Summary

Adds a GitHub Actions workflow to publish the extension to the Open VSX Registry when a version tag is pushed.

This addresses #154 - the extension is currently outdated on Open VSX (v0.1.6) while the VS Code Marketplace has v2.2.0+. Editors that use Open VSX as their extension registry (such as Cursor, VSCodium, and Gitpod) are stuck on the old version, which is missing theme configuration settings like colorTheme, lightTheme, and darkTheme.

What this adds

• .github/workflows/publish-open-vsx.yml - triggered on version tags (v*) and manual dispatch
• Uses HaaLeo/publish-vscode-extension@v2 which handles packaging and publishing via the ovsx CLI
• skipDuplicate: true to safely handle re-runs

Setup required

To enable the workflow, an OPEN_VSX_TOKEN secret needs to be added to the repository settings:

1. Log into open-vsx.org with your GitHub account
2. Go to profile settings > Access Tokens > Generate New Token
3. Add the token as a repository secret named OPEN_VSX_TOKEN
4. Ensure the bierner namespace is claimed on Open VSX (run npx ovsx create-namespace bierner -p <token> if needed)

Closes #154