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/config/prefs.rs

@@ -457,6 +457,9 @@ mod gen {
                     enabled: bool,
                 },
                 legacy_layout: bool,
+                tables: {
+                    enabled: bool,
+                },
                 #[serde(default = "default_layout_threads")]
                 threads: i64,
                 writing_mode: {