This change removes premature reset of
`block_container_y_position_update_callback`. Also makes callback
private in `BlockMarginState`, because resetting it independently of
currently accumulated margins is incorrect.
Lots of test expectations are updated, but there is no visual
difference.
Fixes https://github.com/LadybirdBrowser/ladybird/issues/6074
Inline nodes in our layout tree have a position, so let's show it. By
centralizing the logic for this, block nodes now lose their redundant
'content-size' dump info which is already part of the box model dump.
