teach 'create from usage' to add 'operator' modifier when necessary

idea/testData/quickfix/createFromUsage/createFunction/binaryOperations/plusMissingArgs.kt.after

@@ -3,7 +3,7 @@
 class A<T>(val n: T) {
     fun plus(i: Int, s: String): A<T> = throw Exception()
 
-    fun plus(t: T): A<T> {
+    operator fun plus(t: T): A<T> {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }