Warn if a serialized field is being excluded from build with #if UNITY_EDITOR

Unity doesn't like when the serialized data layout changes between the editor and builds.
For example, having something like this in a behavior `SomeBehaviour`

```
#if UNITY_EDITOR
[SerializeField]
private int _someField;
#endif
```

or this

```
#if UNITY_EDITOR
public int SomeField;
#endif
```

will cause problems in builds (A script behavior (probably SomeBehavior) has a different serialization layout when loading. (Read X bytes but expected Y bytes))

Having a warning (or error) for those situations would be pretty nice.