Support Display::Contents

[nicoburns]

What problem does this solve or what need does it fill?

Display::Contents is useful when you want to toggle between layouts which require different tree structures without actually altering the tree structure.

Requested in bevyengine/bevy#9731

What solution would you like?

• Add a Display::Contents variant to the Display enum
• Alter the children iterator of the Taffy tree implementation to check each Node's Display property, an recurse into children that are set to Display::Contents, returning those grandchildren (and great-grandchildren, etc in the case that any of the grandchild nodes are themselves set to Display::Contents instead of the child itself.
• Process the Display::Contents child itself by setting it to a zero layout.
• I believe that this can be implemented entirely in the tree/storage layer (and the "dispatch" function that switches between algorithms) and the layout algorithm's themselves should not need any changes.

What alternative(s) have you considered?

Don't support Display::Contents and require users of Taffy to alter their tree structure if they want to achieve a similar effect.

Additional context

As the implementation is in the tree/storage layer, users of Taffy implementing custom tree/storage layers will have to implement this themselves. Implementing Display::Contents also requires the tree/storage layer to be able to access non-direct child nodes, so custom tree/storage that can only access direct children may not be to implement this.