Add support for public and internal ProxyClass (#58)

* x

* fx

* .

tests/ProxyInterfaceSourceGeneratorTests/Source/ITestClassInternal.cs

@@ -0,0 +1,6 @@
+namespace ProxyInterfaceSourceGeneratorTests.Source
+{
+    public partial interface ITestClassInternal
+    {
+    }
+}

tests/ProxyInterfaceSourceGeneratorTests/Source/TestClassInternal.cs

@@ -0,0 +1,7 @@
+namespace ProxyInterfaceSourceGeneratorTests.Source
+{
+    public class TestClassInternal
+    {
+        public bool Test { get; set; }
+    }
+}