Add support for indexers (#37)

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* ok

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tests/ProxyInterfaceSourceGeneratorTests/Destination/ProxyInterfaceSourceGeneratorTests.Source.IPerson.g.cs

@@ -14,6 +14,10 @@ namespace ProxyInterfaceSourceGeneratorTests.Source
 {
     public partial interface IPerson
     {
+        ProxyInterfaceSourceGeneratorTests.Source.MyStruct this[int i] { get; set; }
+
+        ProxyInterfaceSourceGeneratorTests.Source.MyStruct this[int i, string s] { get; set; }
+
         string Name { get; set; }
 
         string? StringNullable { get; set; }

tests/ProxyInterfaceSourceGeneratorTests/Destination/ProxyInterfaceSourceGeneratorTests.Source.PersonProxy.g.cs

@@ -19,6 +19,10 @@ namespace ProxyInterfaceSourceGeneratorTests.Source
         public ProxyInterfaceSourceGeneratorTests.Source.Human _InstanceBase { get; }
 
 
+        public ProxyInterfaceSourceGeneratorTests.Source.MyStruct this[int i] { get => _Instance[i]; set => _Instance[i] = value; }
+
+        public ProxyInterfaceSourceGeneratorTests.Source.MyStruct this[int i, string s] { get => _Instance[i, s]; set => _Instance[i, s] = value; }
+
         public string Name { get => _Instance.Name; set => _Instance.Name = value; }
 
         public string? StringNullable { get => _Instance.StringNullable; set => _Instance.StringNullable = value; }