LibWeb: Add Layout::LineBuilder class for incremental line box layout

This class will be used to place items on lines incrementally instead of
the current two-phase approach.

Userland/Libraries/LibWeb/Layout/LineBox.h

@@ -32,6 +32,8 @@ public:
 private:
     friend class BlockContainer;
     friend class InlineFormattingContext;
+    friend class LineBuilder;
+
     NonnullOwnPtrVector<LineBoxFragment> m_fragments;
     float m_width { 0 };
 };