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/flexbox/construct.rs

@@ -165,6 +165,7 @@ where
                 } => {
                     let display_inside = match display {
                         DisplayGeneratingBox::OutsideInside { inside, .. } => inside,
+                        DisplayGeneratingBox::LayoutInternal(_) => display.display_inside(),
                     };
                     let box_ = if info.style.get_box().position.is_absolutely_positioned() {
                         // https://drafts.csswg.org/css-flexbox/#abspos-items