Publish this extension to the OpenVSX marketplace

It'd be great to have this extension on the OpenVSX marketplace, to be directly installable from VSCode-compatible editors like [Theia](https://github.com/eclipse-theia/theia) or from VSCode clones like [VSCodium](https://github.com/VSCodium/vscodium), since the Visual Studio Code marketplace can arguably only be used by official releases.

#79 already made the VSIX files available on the Github releases, which helps installing them or including them in custom Theia-based editors, but this would make the extension appear on the extensions UI for such editors.

The issue points to the existing VSIX release work in #79 and asks for publication on the OpenVSX marketplace. Start by reviewing how releases currently produce and distribute VSIX files, then check the OpenVSX publishing requirements. Done means the extension appears in the marketplace and can be installed through compatible editors such as Theia or VSCodium.

Written by the indexing model from the issue text.