Allow transformation from core/columns

[aurooba]

This idea surfaced in #11. Since columns are essentially containers for blocks like Super List Items are, this is a logical transformation. Ideally, you can select multiple core/columns blocks that you may have set up to create the appearance of a grid, and transform it into a single Super List Block, with each core/column being a Super List Item, then you can truly create a grid. :)

I haven't worked much with transforms, so I'm excited to do this one!