Support multi-platform extensions on open-vsx

[deemp]

Hi! I got problems when trying to fetch this multi-platform extension. Had to skip it eventually. Here's an issue. Could you please add a set of attributes for each platform (even so far unsupported by Nix)? Like extension.src.arm64 = .... P.S. I'm not sure I understand how the multi-platform extensions are actually packaged.