[JS] Fix some JS prettyprinting

- Print a newline after `default` case in `switch`es
- Print a newline after `if` blocks without corresponding `else` blocks

js/js.ast/src/org/jetbrains/kotlin/js/backend/JsToStringGenerationVisitor.java

@@ -438,6 +438,7 @@ public class JsToStringGenerationVisitor extends JsVisitor {
         _colon();
 
         popSourceInfo();
+        newlineOpt();
 
         sourceLocationConsumer.pushSourceInfo(null);
         printSwitchMemberStatements(x);
@@ -708,7 +709,7 @@ public class JsToStringGenerationVisitor extends JsVisitor {
         }
         nestedPush(thenStmt);
 
-        if (thenStmt instanceof JsBlock) {
+        if (thenStmt instanceof JsBlock && elseStatement != null) {
             lineBreakAfterBlock = false;
         }