AK: Implement FlyString for the new String class

This implements a FlyString that will de-duplicate String instances. The
FlyString will store the raw encoded data of the String instance: If the
String is a short string, FlyString holds the String::ShortString bytes;
otherwise FlyString holds a pointer to the Detail::StringData.

FlyString itself does not know about String's storage or how to refcount
its Detail::StringData. It defers to String to implement these details.

AK/Forward.h

@@ -21,6 +21,7 @@ class Bitmap;
 using ByteBuffer = Detail::ByteBuffer<32>;
 class CircularBuffer;
 class Error;
+class FlyString;
 class GenericLexer;
 class IPv4Address;
 class JsonArray;