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

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

[Step 10](https://www.w3.org/TR/css-flexbox-1/#algo-visibility) of the flex layout algorithm is currently not implemented. This handles `visibility:collapse` items, which is currently not supported.

## What solution would you like?

Implementing step 10 according to [the specification](https://www.w3.org/TR/css-flexbox-1/#algo-visibility).

## What alternative(s) have you considered?

We could choose to not support `visibility:collapse`.

## Additional context

Related to #39.

Start with src/flexbox.rs lines 1615-1628 and read Step 10 of the CSS Flexible Box Layout specification. Review related issue #39 for context. Done means the flex layout algorithm implements the specified handling for visibility:collapse items, which is currently unsupported.

Written by the indexing model from the issue text.