Publish extension to open-vsx

[alt-romes]

@BinderDavid suggests we publish the extension to open-vsx as well, since vscode extensions cannot be used by certain related programs.

See #54 (comment)

[BinderDavid]

To be precise: The exactly same binary of the extension, i.e. the *.vsix file, is perfectly usable by other editors such as VSCodium. The problem is that the terms and conditions of the Microsoft VSCode marketplace disallow other programs to download extensions from there. open-vsx hosts exactly the same files as the Microsoft marketplace, but without the artificial restriction that only VSCode can download from there.