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/LineBoxFragment.cpp

@@ -30,6 +30,7 @@
 #include <LibWeb/Layout/LayoutText.h>
 #include <LibWeb/Layout/LineBoxFragment.h>
 #include <LibWeb/RenderingContext.h>
+#include <ctype.h>
 
 namespace Web {
 
@@ -45,6 +46,14 @@ void LineBoxFragment::render(RenderingContext& context)
     }
 }
 
+bool LineBoxFragment::ends_in_whitespace() const
+{
+    auto text = this->text();
+    if (text.is_empty())
+        return false;
+    return isspace(text[text.length() - 1]);
+}
+
 bool LineBoxFragment::is_justifiable_whitespace() const
 {
     return text() == " ";