Add initial support for table box tree construction (#30799)

This is the first part of constructing the box tree for table layout. No
layout is actually done and the construction of tables is now hidden
behind a flag (in order to not regress WPT).  Notably, this does not
handle anonymous table part construction, when the DOM does not reflect
a fully-formed table. That's part two.

Progress toward #27459.

Co-authored-by: Oriol Brufau <obrufau@igalia.com>
Co-authored-by: Manish Goregaokar <manishsmail@gmail.com>

components/layout_2020/flow/construct.rs

@@ -273,6 +273,9 @@ where
                     }
                 },
             },
+            DisplayGeneratingBox::LayoutInternal(_) => {
+                unreachable!("The result of blockification should never be layout-internal value.");
+            },
         }
     }
 

components/layout_2020/flow/inline.rs

@@ -2248,6 +2248,11 @@ impl AbsolutelyPositionedLineItem {
                     block: Length::zero(),
                 }
             },
+            Display::GeneratingBox(DisplayGeneratingBox::LayoutInternal(_)) => {
+                unreachable!(
+                    "The result of blockification should never be a layout-internal value."
+                );
+            },
             Display::Contents => {
                 panic!("display:contents does not generate an abspos box")
             },

components/layout_2020/flow/root.rs

@@ -225,7 +225,7 @@ fn construct_for_root_element<'dom>(
             unreachable!()
         },
         // The root element is blockified, ignore DisplayOutside
-        Display::GeneratingBox(DisplayGeneratingBox::OutsideInside { inside, .. }) => inside,
+        Display::GeneratingBox(display_generating_box) => display_generating_box.display_inside(),
     };
 
     let contents =