I follow this GitHub link in the V6 version of CodeMirror which took me to this repo: GitHub - codemirror/codemirror.next: The next generation of the CodeMirror in-browser editor. But it seems that this is not the source code of CodeMirror itself. I think it is sort of example code or starter kit.
Each of the packages has its own repository, with rather easy-to-guess names—@codemirror/view is github.com/codemirror/view, etc