Add OVSX publish step to release workflow

[alaycock-stripe]

Closes markdoc/markdoc#595

Uses the ovsx package to publish to the Open VSX Marketplace to make the extension available in VS Code compatible editors, such as Cursor.

You'll need to signup, sign the publisher agreement, and get an access token. Docs here.

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

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