Support Display::FlowRoot

[nicoburns]

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

display: flow-root nodes use the same algorithm as CSS Block layout, except that they force the creation of a new https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Block_formatting_context. In practice the main impact of this will be to prevent margin collapsing between the Display::FlowRoot node and it's children, or through the Display::FlowRoot node. If in future we support floats, it would also affect those.

What solution would you like?

Implement support for Display::FlowRoot in Taffy, by adding a new variant to the Display enum and updating the Block Layout implementation to work with it.