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.");
+            },
         }
     }