[css-grid] Decide on a name for `item-slack`

[Loirooriol]

#9328 (comment) didn't resolve on the name for this property.

In the spec it has been tentatively added as masonry-slack: https://drafts.csswg.org/css-grid-3/#masonry-slack

Other ideas from #9328 are masonry-threshold, masonry-placement-threshold, masonry-strictness, masonry-adjust, masonry-skip, masonry-sensitivity.

[Loirooriol]

I'm liking masonry-tolerance (as in https://en.wikipedia.org/wiki/Engineering_tolerance).

I'm not fond of "slack" because as a non-native English speaker that's a word that I learned considerably later than the others.

"strictness" and "sensitivity" can seem to imply that a higher value will choose the track by comparing the running positions more strictly, while it's actually the opposite.