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

idea/testData/quickfix/createFromUsage/createFunction/invoke/invokeOnLibType.kt.after

@@ -6,6 +6,6 @@ fun test(): A<String> {
     return 1(2, "2")
 }
 
-fun Int.invoke(i: Int, s: String): A<String> {
+operator fun Int.invoke(i: Int, s: String): A<String> {
     throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
 }

idea/testData/quickfix/createFromUsage/createFunction/invoke/invokeOnUserType.kt.after

@@ -1,7 +1,7 @@
 // "Create member function 'invoke'" "true"
 
 class A<T>(val n: T) {
-    fun invoke(t: T, s: String): B<String> {
+    operator fun invoke(t: T, s: String): B<String> {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }

idea/testData/quickfix/createFromUsage/createFunction/invoke/invokeOnUserTypeWithTypeParams.kt.after

@@ -1,7 +1,7 @@
 // "Create member function 'invoke'" "true"
 
 class A<T>(val n: T) {
-    fun <V> invoke(u: T, s: String): B<V> {
+    operator fun <V> invoke(u: T, s: String): B<V> {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }

idea/testData/quickfix/createFromUsage/createFunction/invoke/invokeWithExplicitParamNamesOnUserType.kt.after

@@ -1,7 +1,7 @@
 // "Create member function 'invoke'" "true"
 
 class A<T>(val n: T) {
-    fun invoke(abc: T, ghi: A<T>, def: String): A<T> {
+    operator fun invoke(abc: T, ghi: A<T>, def: String): A<T> {
         throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
     }
 }