`maxGraph` was using `lerna` v4 to build and only used its "build
workspaces" support, not the "publishing" support as we only published a
single package and it is done manually, without using `lerna`.
The support for packages in `lerna` was removed in version 7. See
https://github.com/lerna/lerna/releases/tag/7.0.0 and
https://lerna.js.org/docs/legacy-package-management.
So, start using npm workspaces as we don't need to manage the publishing
of several packages for now.
This will allow to do more reproducible builds by locking dependencies
with a `package-lock.json` file. This didn't work well with `lerna` v4
and had been disabled.
Thomas Bouffard