I don't think it makes sense to have a tag for the shell builtin source specifically. It's not that special a feature that it wouldn't just fit under shell-script (or bash, which is what most of those are tagged with anyway). Besides, source isn't even a standard command. For POSIX-compatibility, it should be dot-command instead.
It would probably be best to make source-code the main tag and source an alias to make it more clear what it's about.