Switch Lerna to independent mode

[foolip]

https://github.com/lerna/lerna#independent-mode

The intention is for a @webref/css package to be versioned independently
of @webref/idl. In the meantime, this avoids two places for the same
version.

[foolip]

There's a consequence of this which is worth pointing out. In the default mode, all packages get the same version, and npx lerna version patch would create a tag called v1.0.1. With independent mode, each package would get its own tag, so we'd get @webref/idl@1.0.1 instead.

@dontcallmedom @tidoust do you think we should tag the releases at all? If so do you care if there are more tags in total? There is something to be said for the scoped tags, since it's not webref itself which will be at v1.0.1.

[foolip]

Regarding tagging, I went ahead and retroactively tagged @webref/idl@1.0.0 on the branch which used to release. I can delete it again if you'd rather not have any tags for releases.

[tidoust]

Makes sense to me to have tags per package release.