Implement step 10 of the flex layout algorithm ("Collapse `visibility:collapse` items")

[TimJentzsch]

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

Step 10 of the flex layout algorithm is currently not implemented. This handles visibility:collapse items, which is currently not supported.

https://github.com/DioxusLabs/sprawl/blob/11701ee97d68b22a231cac081aab4308af136706/src/flexbox.rs#L1615-L1628

What solution would you like?

Implementing step 10 according to the specification.

What alternative(s) have you considered?

We could choose to not support visibility:collapse.

Additional context

Related to #39.