[css-grid] 'auto' keyword should work with name grid lines/areas

[plinss]

Consider the sample grid:

grid-template-columns: 1fr repeat(4, [icon-start] auto [icon-end] 1fr);
grid-template-rows: [icon-start] auto [icon-end];

With an image:

img { grid-area: icon; }

And the markup:

<img src=icon1.png>
<img src=icon2.png>
<img src=icon3.png>
<img src=icon4.png>

All four images will be placed into the first 'icon' grid area, and require specific style rules to target the other grid areas. It would be useful to be able to use the auto placement algorithm but target specific named grid lines/areas, e.g:

img { grid-area: icon auto; }

and have each image be placed in the first unoccupied 'icon' grid area.