layout: Collect both start and end baselines for fragments (#31230)

This change starts collecting the starting baseline set for fragments,
which is necessary for some layout modes (flex and tables, namely) as
well as being important for the implementation of `align-items`. In
addition, it converts baseline measurement to use `Au` everywhere.

Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/positioned.rs

@@ -706,7 +706,6 @@ impl HoistedAbsolutelyPositionedBox {
                 None, /* clearance */
                 // We do not set the baseline offset, because absolutely positioned
                 // elements are not inflow.
-                None, /* last_inflow_baseline_offset */
                 CollapsedBlockMargins::zero(),
                 physical_overconstrained,
             )