Deprecate extension annotation in favor of Extension

idea/idea-completion/testData/basic/multifile/ExcludedJavaClass/somePackage/ExctraClass.java

@@ -0,0 +1,6 @@
+package somePackage;
+
+// Such strange name here needed to make completion prefix not intersecting with kotlin.Extension
+public class ExctraClass {
+
+}