Reverted ModelType property to be virtual

While the recent update to avoid read-only `virtual` properties (532c49d) generally makes sense, and should be maintained for most types, it doesn't make sense for `ModelType` as some implementations of this actually need to be dynamic, as they conditionally rely on the _current_ value from `Attributes`. As the `Attributes` aren't populated until after the constructor has initialized the object, that results in the `ModelType` never being updated based on these values. Oops!

Author: JeremyCaney

OnTopic/Metadata/AttributeDescriptor.cs

@@ -90,7 +90,7 @@ public AttributeDescriptor(
     ///   reduces these down into a single type based on how they're exposed in the Topic Library, not based on how they're
     ///   exposed in the editor.
     /// </remarks>
-    public ModelType ModelType { get; protected init; } = ModelType.ScalarValue;
+    public virtual ModelType ModelType { get; protected init; } = ModelType.ScalarValue;
 
     /*==========================================================================================================================
     | PROPERTY: EDITOR TYPE