LibWeb: Rename LayoutNode classes and move them into Layout namespace

Bring the names of various boxes closer to spec language. This should
hopefully make things easier to understand and hack on. :^)

Some notable changes:

- LayoutNode -> Layout::Node
- LayoutBox -> Layout::Box
- LayoutBlock -> Layout::BlockBox
- LayoutReplaced -> Layout::ReplacedBox
- LayoutDocument -> Layout::InitialContainingBlockBox
- LayoutText -> Layout::TextNode
- LayoutInline -> Layout::InlineNode

Note that this is not strictly a "box tree" as we also hang inline/text
nodes in the same tree, and they don't generate boxes. (Instead, they
contribute line box fragments to their containing block!)

Libraries/LibWeb/Layout/LayoutPosition.h

@@ -28,12 +28,12 @@
 
 #include <AK/RefPtr.h>
 
-namespace Web {
+namespace Web::Layout {
 
-class LayoutNode;
+class Node;
 
 struct LayoutPosition {
-    RefPtr<LayoutNode> layout_node;
+    RefPtr<Node> layout_node;
     int index_in_node { 0 };
 };