LibWeb: Show formatting context roots in layout tree dumps

This patch does three things:

- Factors out the code that determines whether a box will create a new
  formatting context for its children (and which type of context)

- Uses that code to mark all formatting context roots in layout tree
  dumps. This makes it much easier to follow along with layout since
  you can now see exactly where control is transferred to a new
  formatting context.

- Rebaselines all existing layout tests, since the output format has
  changed slightly.

Userland/Libraries/LibWeb/Layout/FormattingContext.h

@@ -24,6 +24,8 @@ public:
         Grid,
         Table,
         SVG,
+        InternalReplaced, // Internal hack formatting context for replaced elements. FIXME: Get rid of this.
+        InternalDummy,    // Internal hack formatting context for unimplemented things. FIXME: Get rid of this.
     };
 
     virtual void run(Box const&, LayoutMode, AvailableSpace const&) = 0;
@@ -44,6 +46,8 @@ public:
 
     virtual bool inhibits_floating() const { return false; }
 
+    [[nodiscard]] static Optional<Type> formatting_context_type_created_by_box(Box const&);
+
     static bool creates_block_formatting_context(Box const&);
 
     static CSSPixels compute_width_for_replaced_element(LayoutState const&, ReplacedBox const&, AvailableSpace const&);