[css-grid-3][masonry] Make `item-tolerance` initial value `normal`

[ethanjv]

https://drafts.csswg.org/css-grid-3/#item-slack

With the latest resolution of using item-tolerance and making the item-* properties work differently in multiple layout types, it would make sense to change the initial value of this property to normal, following the similar behavior of [column|row]-gap.

Currently, only masonry has a use for item-tolerance, but it could potentially be reused elsewhere, while the normal behavior for masonry container can be kept as 1em for placement ties.

[Loirooriol]

I agree the initial value should be normal if the default behavior in masonry is 1em or something different than zero.

But normal can be superfluous if it ends up always behaving as 0px. So it may be better to settle #10882 first.