Add Community provided Visual Studio Code AppImage to feed.json

[cadamsdev]

Since Microsoft will not provide an official AppImage for Visual Studio Code
microsoft/vscode#10857

Can we add an unofficial AppImage to the feed.json
Microsoft doesn't have an issue with us doing this. See here
Flatpak is currently doing this so we should too.

I was thinking the unofficial AppImage would come from someone reputable in the AppImage community perhaps an AppImage developer.

@probonopd What do you think? I saw you made an AppImage for this already in the above issue? See here

[probonopd]

The AppImage project does not want to package other people's software. In general we want to encourage upstream packaging (and support). Thus, we help application projects interested in making their own AppImages.

Since the Visual Studio Code team does not want to provide AppImages (yet), we helped another project which does: VSCodium. #1436