Support `Overflow::Auto`

If a node has overflow: auto in a given axis then it should be laid out as normal, but if it overflows then space for a scrollbar should be reserved (as with overflow: scroll) and layout for that node should be redone with the space for the scrollbar taken into account.

Start by locating the layout code that handles node overflow and the existing behavior for overflow: scroll. Trace how an overflowing axis is detected and how a second layout pass could account for scrollbar space; done means overflow: auto lays out normally when content fits and reserves scrollbar space before relayout when it does not.

Written by the indexing model from the issue text.