JS_WriteObject does not serialize symbol keys

It doesn't throw an exception, it just silently ignores them:
```
qjs > new Uint8Array(bjson.write({[Symbol.toStringTag]: "42"}))
Uint8Array(4) [ 12, 0, 8, 0 ]
```
(8 = object tag, 0 = property count)

Labeling bug because preferably quickjs is taught to (de)serialize them. If that's not feasible, it should probably raise an exception.