Add support for SerializeReference

[citizenmatt]

New in Unity 2019.3.

• Applied to a field
• Applied only to reference types - the field is serialised as a reference, not a value, allowing for e.g. expressing object graphs

Initial thoughts:

• Means implicit use + assignment (like [SerializeField])
• Need to treat these fields as serialised fields (naming standards, in use, context actions, code vision)
• Update "make serialised field" context actions - make serialised field/make serialised references
• Update sprop and sfield Live Templates? Allow choosing attribute?
• Only support in Unity 2019.3+

See more details in the Unity forum

[citizenmatt]

If this supports polymorphic serialisation, it might be useful to highlight when a subtype isn't [Serializable]

[citizenmatt]

Other possible issues:

• Check if things break if the class is renamed - the class name might be part of the serialised data
• It looks like the serialised object can't derive from UnityEngine.Object. If so, add a warning + "Why is Rider suggesting this?" for this scenario.

[aybe]

There's a bug in Rider 2020.2.4:

TrackPerk is abstract, it is and all its derived marked as [Serializable].

The bug is as follows,

I wanted to add [HideInInspector] but it's marked as useless, tried to add [SerializeField] but it's also marked as useless.