Added support for staggered grid

[bpappin]

The internal structure of LazyStaggeredGridItemScope is missing the animateItemPlacement() modifier that LazyGridItemScope has, so that section does not work properly.

Also, there was no way to tell the orientation from LazyStaggeredGridState.isVertical because its internal and not accessible, which lead to separate remember clauses.

[IHNEL]

will someone merge this?

[kafri8889]

Please merge this @aclassen

[bpappin]

For those needing this, it looks like its been added for 0.9.7, but the PR not merged. As far as i can tell, 0.9.7 has not been published.