Add warning for ExecuteAlways on wrong target

Unity has several attributes that do not specify a target, but are only used on a specific entity - e.g. `[SerializedField]` is only applicable to fields, but can be applied to classes and methods. We have an inspection for this to show a warning if an attribute is applied to the wrong target. Add support for `[ExecuteAlways]` ([introduced in 2018.3](https://ngtools.tech/uv/UnityEngine.ExecuteAlways/)). It should be applied to classes only.