Create from Usage: Add operator modifier to indexing get/set functions

#KT-10185 Fixed

idea/testData/quickfix/createFromUsage/createFunction/set/createSetFromUsage1.kt.after

@@ -4,7 +4,7 @@ class Foo<T> {
         y["", w] = w
     }
 
-    private fun set(s: String, w: T, value: T) {
+    private operator fun set(s: String, w: T, value: T) {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }