Add support for positioning child blocks inside a Grid block.

[isabel_brison]

It should be possible to position a block in a specific grid cell inside a Grid block or block with grid layout.

The following Gutenberg PRs are relevant to this feature:

​https://github.com/WordPress/gutenberg/pull/59483
​https://github.com/WordPress/gutenberg/pull/60652
​https://github.com/WordPress/gutenberg/pull/62777

Trac ticket: https://core.trac.wordpress.org/ticket/61505

Syncs changes from the following Gutenberg PRs:

​https://github.com/WordPress/gutenberg/pull/59483
​https://github.com/WordPress/gutenberg/pull/60652
​https://github.com/WordPress/gutenberg/pull/62777

### Note: this changeset should only be committed once the feature is stabilised in Gutenberg!

This feature won't land in 6.7 so no need to commit this just yet.

I've create a PR in the Gutenberg repo to relocate the changelog notes, ​https://github.com/WordPress/gutenberg/pull/64991

This has a few extra PRs listed against it in the merge ticket ​https://github.com/WordPress/gutenberg/issues/68712

As this hasn't changed since September, it will need an update. cc @Mamaduka

@peterwilsoncc, I think we need to bump this to the next major release again.

I checked with @tellthemachines; these features are still behind the experimental flag and not ready for prime time.

@Mamaduka Thanks George, should I move the backport file the the changelog 6.9 folder? The PHP in the gutenberg plugin isn't in the experimental folder so it doesn't appear I can delete the file until it's ready to merge.

@peterwilsoncc, here's relocation PR: ​https://github.com/WordPress/gutenberg/pull/69068.

I would like to confirm the status of this PR. The current contents of wp_get_layout_style() and gutenberg_get_layout_style() are almost the same, so I believe it is no longer necessary to apply this PR. What do you think?

Thanks for checking @t-hamano! Yes I think we can close this one.

[isabel_brison]

I'm closing this as some of the changes tracked here are already in core as part of other features, and there's currently no timeline for the manual positioning work to be stabilised.