LibJS: Add a simple reference-counted UTF-16 string

To help alleviate memory usage when creating and copying large strings,
create a simple wrapper around a Vector<u16> to reference count UTF-16
strings.

Userland/Libraries/LibJS/Forward.h

@@ -169,6 +169,7 @@ class Statement;
 class StringOrSymbol;
 class Symbol;
 class Token;
+class Utf16String;
 class VM;
 class Value;
 class WeakContainer;