LibHTML: Introduce LayoutBox and LayoutNodeWithStyleAndBoxModelMetrics

To streamline the layout tree and remove irrelevant data from classes
that don't need it, this patch adds two new LayoutNode subclasses.

LayoutNodeWithStyleAndBoxModelMetrics should be inherited by any layout
node that cares about box model metrics (margin, border, and padding.)
LayoutBox should be inherited by any layout node that can have a rect.

This makes LayoutText significantly smaller (from 140 to 40 bytes) and
clarifies a lot of things about the layout tree.

I'm also adding next_sibling() and previous_sibling() overloads to
LayoutBlock that return a LayoutBlock*. This is okay since blocks only
ever have block siblings.

Do also note that the semantics of is<T> slightly change in this patch:
is<T>(nullptr) now returns true, to facilitate allowing to<T>(nullptr).

Libraries/LibHTML/Layout/LayoutBreak.cpp

@@ -2,7 +2,7 @@
 #include <LibHTML/Layout/LayoutBreak.h>
 
 LayoutBreak::LayoutBreak(const HTMLBRElement& element)
-    : LayoutNode(&element)
+    : LayoutNodeWithStyleAndBoxModelMetrics(&element, StyleProperties::create())
 {
     set_inline(true);
 }