Simplify regex for checking valid property reference

com.unity.shadergraph/Editor/Data/Graphs/AbstractMaterialGraph.cs

             if (!referenceName.StartsWith("_"))
                 referenceName = "_" + referenceName;
 
-            referenceName = Regex.Replace(referenceName, @"\s+", "_");
-            referenceName = Regex.Replace(referenceName, @"^[^A-Za-z_]", "_");
-            referenceName = Regex.Replace(referenceName, @"[^A-Za-z_0-9]", "_");
+            referenceName = Regex.Replace(referenceName, @"(?:[^A-Za-z_0-9])|(?:\s)", "_");
 
             // Similar to the property names, duplicate names are handled by appending `_n`
             // to the end of the referene name.

com.unity.shadergraph/Editor/Drawing/Blackboard/BlackboardFieldPropertyView.cs

             m_Graph = graph;
             m_Property = property;
 
-            m_ReferenceNameField = new TextField(30, false, false, ' ');
+            m_ReferenceNameField = new TextField(512, false, false, ' ');
             AddRow("Reference", m_ReferenceNameField);
             m_ReferenceNameField.value = property.referenceName;
             m_ReferenceNameField.OnValueChanged(newName =>