[css-grid] Mark some cells as busy so they are skip in auto-placement

[mrego]

This idea was already introduced by @jensimmons long time ago at TPAC 2016: https://speakerdeck.com/jensimmons/proposal-to-csswg-sept-2016?slide=51
But I'm just reporting the issue here to track it for future work/proposals around CSS Grid Layout.

I guess it'd be nice to have some kind of syntax that allows us to mark some cells as busy (even without the need to place there empty dummy elements), and the auto-placement algorithm would skip them.

Of course we need to think also in a syntax, so we can refer to a list of cells not a single one.

[ByteEater-pl]

This could even be specified per element. Example syntax:

allow-placement: <selector>

[fantasai]

Might end up addressed by #499