Support CSS Block layout (Display::Block)

[nicoburns]

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

While it is a quirky layout mode that's not ideally suited to application layout, it is commonly used in web layouts and is a necessary for anyone wishing to do full HTML/CSS layout accurately.

What solution would you like?

Implement support for Display::Block in Taffy:

• https://www.w3.org/TR/CSS22/visuren.html#block-formatting
• https://www.w3.org/TR/CSS22/box.html#collapsing-margins

Additional context

This may potentially clash with some of the proposed changes in #326 as Block layout seems to require access to the styles of descendent nodes that are not direct children. However, this would also be required for Subgrid layout, and I think it can be resolved.