LibJS: Let Utf16String be forward-declared in Value.h

It's only used as a template parameter, so let it be forward-declared.
Otherwise, we aren't able to include Completion.h in Utf16String.h, as
there would be a Utf16String -> Completion -> Value -> Utf16String
include cycle.

Userland/Libraries/LibJS/Runtime/Value.cpp

@@ -36,6 +36,7 @@
 #include <LibJS/Runtime/StringObject.h>
 #include <LibJS/Runtime/StringPrototype.h>
 #include <LibJS/Runtime/SymbolObject.h>
+#include <LibJS/Runtime/Utf16String.h>
 #include <LibJS/Runtime/VM.h>
 #include <LibJS/Runtime/Value.h>
 #include <math.h>

Userland/Libraries/LibJS/Runtime/Value.h

@@ -19,7 +19,6 @@
 #include <LibJS/Forward.h>
 #include <LibJS/Heap/GCPtr.h>
 #include <LibJS/Runtime/BigInt.h>
-#include <LibJS/Runtime/Utf16String.h>
 #include <math.h>
 
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