Feature: Extend for mixins (microsoft#2096)

Co-authored-by: saschanaz <saschanaz@users.noreply.github.com>

Author: Bashamega

inputfiles/overridingTypes.jsonc

@@ -19,12 +19,6 @@
                     }
                 }
             },
-            "ChildNode": {
-                "extends": "Node"
-            },
-            "ParentNode": {
-                "extends": "Node"
-            },
             "Animatable": {
                 "methods": {
                     "method": {

inputfiles/patches/child-parent-node.kdl

@@ -0,0 +1,3 @@
+// ChildNode and ParentNode are actually defined as mixins, but because of their names they have historically been used as a sub-interface of Node.
+interface-mixin ChildNode extends=Node
+interface-mixin ParentNode extends=Node

src/build/patches.ts

@@ -92,11 +92,15 @@ function handleMixin(node: Node): DeepPartial<Interface> {
     }
   }
 
-  return {
+  const result = {
     name,
     events: { event },
     properties: { property },
   } as DeepPartial<Interface>;
+  if (node.properties.extends) {
+    result.extends = node.properties.extends as string;
+  }
+  return result;
 }
 
 /**