Cross publish to the open VSX registry

[UnwarySage]

Was hoping to use this, but it is not available to code-oss users.
Here's a better explanation of why this is important, but the tl;dr is that using the open source version of VSCode and trying to access open source extensions goes through the closed source microsoft servers, and is illegal, but alternatives such as open-VSX exist.

[omentic]

(duplicate of #176)