Grid: Remove grid positioning attributes when you move a block outside of the grid container

[noisysocks]

When a Grid block is set to Manual mode, children of the grid are positioned according to attributes on each child block.

There are four:

• style.layout.columnStart - Column that the child begins at (x position)
• style.layout.rowStart - Row that the child begins at (y position)
• style.layout.columnSpan - How many columns wide the child is
• style.layout.rowSpan - How many rows high the child is

Using drag and drop, the movers, resize handles, etc. will adjust these attributes.

If a child is added to a grid and doesn't already contain these attributes, they are added by a hook called useGridLayoutSync.

If the grid is switched from Manual mode to Auto mode, useGridLayoutSync will remove all of these attributes on the children.

We need to implement logic that removes these attributes if the child block is moved out of the grid container. Right now, if you drag and drop a block from inside a grid to some place else, and then view the source, you'll see pointless columnStart etc. attributes on that block.

One way to do this might be to update useGridLayoutSync to keep a list of children it knows about. Then, when a child it knows about is no longer in innerBlocks, but that child is still in the block editor store, we can assume it has been moved outside of the grid and that we should remove the layout attributes.