AK: Rename <AK/AKString.h> to <AK/String.h>

This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.

Let's just not support building that way, so String.h can have an
objectively nicer name. :^)

Libraries/LibHTML/DOM/Element.h

@@ -1,7 +1,7 @@
 #pragma once
 
 #include <LibHTML/DOM/ParentNode.h>
-#include <AK/AKString.h>
+#include <AK/String.h>
 
 class Attribute {
 public: