LibWeb: Ignore some collapsible whitespace when building lines

When collapsing whitespace, we can skip over all-whitespace chunks at
the start of each line, and immediately following fragments that
themselves end in whitespace.

Userland/Libraries/LibWeb/Layout/InlineLevelIterator.h

@@ -32,6 +32,7 @@ public:
         size_t length_in_node { 0 };
         float width { 0.0f };
         bool should_force_break { false };
+        bool is_collapsible_whitespace { false };
     };
 
     explicit InlineLevelIterator(Layout::Box& container, LayoutMode layout_mode)