LibWeb: Teach line layout to collapse whitespace across fragments

This kind of HTML now produces a single piece of whitespace:

<span> </span> <span> </span> <span> </span>

We achieve this by checking if the last fragment on the last line ends
in whitespace. If so, we either don't add a fragment at all (for the
current chunk) or we simply skip over all whitespace at the head of
the current chunk (instead of collapsing it to a single ' '.)

Libraries/LibWeb/Layout/LineBox.h

@@ -45,6 +45,8 @@ public:
 
     void trim_trailing_whitespace();
 
+    bool ends_in_whitespace() const;
+
 private:
     friend class LayoutBlock;
     NonnullOwnPtrVector<LineBoxFragment> m_fragments;