Publish on OpenVSX

This extension is currently available on VisualStudio Marketplace, which can only be legally used by the Microsoft build of VSCode. Since this is a non-free build, I am using [VSCodium](https://vscodium.com/), which is using [OpenVSX](https://open-vsx.org/) as its marketplace.

This extension was [published on OpenVSX](https://open-vsx.org/extension/firefox-devtools/vscode-firefox-debug), but it is unofficial, and it is not up-to-date. The best option would be to follow the [documentation to create a namespace](https://github.com/eclipse/openvsx/wiki/Namespace-Access) and upload the extension there. Of course, this requires some work. There is an [easiest path](https://github.com/open-vsx/publish-extensions), but it should be used by extension users as a wish list.