publish extension to open-vsx registry

[DetachHead]

it would be nice if this extension was also published to https://open-vsx.org/, which is an open source version of the vscode extension marketplace. this would allow the extension to be used in vscode forks such as vscodium.

[RandomEngy]

Looks like you can do a one-time setup to publish automatically: EclipseFdn/publish-extensions#1088

We'll see if it's accepted.