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/lib.rs

@@ -23,6 +23,7 @@ pub mod query;
 mod replaced;
 mod sizing;
 mod style_ext;
+pub mod table;
 pub mod traversal;
 
 pub use flow::BoxTree;