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. :^)

AK/StringView.cpp

@@ -1,4 +1,4 @@
-#include <AK/AKString.h>
+#include <AK/String.h>
 #include <AK/StringView.h>
 
 namespace AK {