Publish to OpenVSX

Currently the extension is not available on [open-vsx.org](https://open-vsx.org/), meaning it can only be installed manually in IDE's such as [VSCodium](https://vscodium.com/) and [Eclipse Theia](https://theia-ide.org/). It would be great if the theme could be packaged and published to OpenVSX for easier installation and greater discoverability on those platforms!