Add export of kotlinx.datetime.format in module-info (#438)

This allows usages of the introduced format api from java 9 modules

Author: cdelabou

core/jvm/java9/module-info.java

@@ -4,4 +4,5 @@
 
     exports kotlinx.datetime;
     exports kotlinx.datetime.serializers;
+    exports kotlinx.datetime.format;
 }