[css-grid-3][masonry] Adjustments/additions to `grid` shorthanding

[fantasai]

Opening this issue to split out discussion from #11243 (comment) on adjustments to shorthanding:

• Any adjustments we might need to make the grid* shorthands accommodate masonry features.
• Possible addition of a masonry-specific shorthand.

(Might need to wait for #11480 to settle before finalizing anything though.)

[alisonmaher]

In Chromium, we have a prototype of a separate masonry shorthand: https://developer.chrome.com/blog/masonry-update#try-masonry (enabling the Masonry feature flag is required to play with it).

Given the two-dimensional nature of Grid, the grid shorthand ends up being fairly confusing for Masonry, given that in masonry, we only care about one direction. Having a separate shorthand that only sets the pieces needed for masonry makes it much clearer what you are setting, and I found in writing tests, leads to less opportunity for error, which I suspect will be the case for authors, as well.