[css-grid] Auto-placement aligning to a named line

I have seen a couple of requests from authors to be able to auto-place to the next available row or column line with a certain name.

This came up in a Q&A that I am running, and there is also an earlier request for this behaviour detailed on the www-style list, and on the wiki for possible L2 features. Raising this issue just to capture these in one place.