[css-grid-3] Should grid-lanes live in the Grid spec, or its own module?

[alisonmaher]

Now that we've resolved to use display: grid-lanes for the masonry layout switch, I wanted to raise an issue to confirm if Grid Lanes should live in css-grid-3 or live in its own module (something like css-grid-lanes-1)?

We are reusing a lot of concepts from Grid, but the Grid spec is already fairly large and complex, so I'm wondering if it will simplify things to move this to its own module given it is a separate display (where each module cross references each other where needed).