AK: Reduce header dependency graph of String.h

String.h no longer pulls in StringView.h. We do this by moving a bunch
of String functions out-of-line.

AK/StringBuilder.cpp

@@ -29,6 +29,7 @@
 #include <AK/StdLibExtras.h>
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
+#include <AK/StringView.h>
 
 namespace AK {