LibJS: Move "strict mode" state to the call stack

Each call frame now knows whether it's executing in strict mode.
It's no longer necessary to access the scope stack to find this mode.

Libraries/LibJS/Runtime/Function.cpp

@@ -57,7 +57,7 @@ BoundFunction* Function::bind(Value bound_this_value, Vector<Value> arguments)
         switch (bound_this_value.type()) {
         case Value::Type::Undefined:
         case Value::Type::Null:
-            if (interpreter().in_strict_mode())
+            if (vm().in_strict_mode())
                 return bound_this_value;
             return &global_object();
         default: