LibJS: Don't return the "last computed value" from Interpreter::run()

Only return whatever a "return" statment told us to return.
The last computed value is now available in Interpreter::last_value()
instead, where the REPL can pick it up.

Libraries/LibJS/Interpreter.h

@@ -154,11 +154,15 @@ public:
         return throw_exception(heap().allocate<Exception>(value));
     }
 
+    Value last_value() const { return m_last_value; }
+
 private:
     Interpreter();
 
     Heap m_heap;
 
+    Value m_last_value;
+
     Vector<ScopeFrame> m_scope_stack;
     Vector<CallFrame> m_call_stack;