Add highlight for serialised field with same name as serialised field in base class

If we have a base class with a private serialised field called `MySerialisedField` and a derived class that also has a serialised field called `MySerialisedField`, then the code will compile, but Unity will fail to serialise the class hierarchy, due to the name clash.

Rider/ReSharper will show a gutter icon and add a warning that the `new` keyword is required if the base class' field is not private. The Unity plugin should add a similar warning when the base class' field is private. We already do a similar thing for event functions. E.g. if the base class defines a private `Update` function and the derived class also defines `Update`, we add the gutter icon (but no warning. We might want to reconsider that).