Add support for using a simple type-name (#22)

src/ProxyInterfaceSourceGenerator/Compatibility/NullableAttributes.cs

@@ -0,0 +1,18 @@
+// https://stackoverflow.com/questions/61573959/how-to-resolve-error-notnullwhen-attribute-is-inaccessible-due-to-its-protectio
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter will not be null.
+        /// </param>
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+}