[css-grid] Removing grid-items from layout calculations in one direction, especially for `auto-fit`

Spec: https://www.w3.org/TR/css-grid-1/#auto-repeat
Codepen: https://codepen.io/pmccloghrylaing/full/PBJNKG/

I raised an issue previously (#2956) about creating a tabbed layout where the tabs were positioned using auto-fit (so they always take up the full width), and the tab panel takes up the full width (using grid-column: 1 / -1). This ended up behaving exactly like auto-fill. One of the advantages of this approach is it falls back to an accordion.

The solution I ended up using was to applying position: absolute to the tab panel so it didn't affect the layout calculations for auto-fit. That worked in my particular case because the tab panel was a fixed height with overflow: auto, but what I would have preferred is a solution that allowed you to define that a grid item occupied particular columns (or rows) but shouldn't be used in layout calculations in that direction, especially for auto-fit.

This could tie in with #2402

Start with the CSS Grid Level 1 auto-repeat section and the linked CodePen, then compare the earlier issue #2956 and related issue #2402. Determine whether the requested directional exclusion for grid-item layout calculations can be specified; done requires a resolved proposal or specification direction from the CSS Working Group.