Block auto margins are zeroed by two auto insets not two auto margins

The rule is:

"If all three of 'left', 'width', and 'right' are 'auto': First set any
'auto' values for 'margin-left' and 'margin-right' to 0."

Taffy was previously erroneously checking 'margin-left' and
'margin-right' instead of 'left' and 'right'

src/compute/block.rs

@@ -383,7 +383,7 @@ fn perform_final_layout_on_in_flow_children(
             let free_x_space = f32_max(0.0, container_inner_width - final_size.width - item_non_auto_x_margin_sum);
             let x_axis_auto_margin_size = {
                 let auto_margin_count = item_margin.left.is_none() as u8 + item_margin.right.is_none() as u8;
-                if auto_margin_count == 2 && item.size.width.is_none() {
+                if item.inset.left.is_auto() && item.inset.right.is_auto() && item.size.width.is_none() {
                     0.0
                 } else if auto_margin_count > 0 {
                     free_x_space / auto_margin_count as f32