Add 'operator' keyword

compiler/testData/loadJava/compiledKotlin/fun/OperatorKeyword.kt

@@ -0,0 +1,5 @@
+package test
+
+class Example {
+    operator fun plus(other: Example) {}
+}

compiler/testData/loadJava/compiledKotlin/fun/OperatorKeyword.txt

@@ -0,0 +1,6 @@
+package test
+
+public final class Example {
+    /*primary*/ public constructor Example()
+    public final operator fun plus(/*0*/ other: test.Example): kotlin.Unit
+}