Add implicit conversion to ProjectInstance (#42)

Missed this in the last commit

Author: jeffkl

src/MSBuildProjectCreator/ProjectCreator.Conversions.cs

@@ -4,6 +4,7 @@
 
 using Microsoft.Build.Construction;
 using Microsoft.Build.Evaluation;
+using Microsoft.Build.Execution;
 using System.Xml.Linq;
 
 namespace Microsoft.Build.Utilities.ProjectCreation
@@ -28,6 +29,15 @@ public static implicit operator ProjectCollection(ProjectCreator creator)
             return creator.ProjectCollection;
         }
 
+        /// <summary>
+        /// Defines an implicit conversion of a <see cref="ProjectCreator"/> to a <see cref="ProjectInstance"/>.
+        /// </summary>
+        /// <param name="creator">A <see cref="ProjectCreator"/> to convert.</param>
+        public static implicit operator ProjectInstance(ProjectCreator creator)
+        {
+            return creator.ProjectInstance;
+        }
+
         /// <summary>
         /// Defines an implicit conversion of a <see cref="ProjectCreator"/> to a <see cref="ProjectRootElement"/>.
         /// </summary>