[JS] Add toString() override to JsPropertyInitializer

This doesn't affect js2string, but is convenient for debugging

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

@@ -59,4 +59,9 @@ public class JsPropertyInitializer extends SourceInfoAwareJsNode {
     public JsPropertyInitializer deepCopy() {
         return new JsPropertyInitializer(labelExpr.deepCopy(), valueExpr.deepCopy()).withMetadataFrom(this);
     }
+
+    @Override
+    public String toString() {
+        return labelExpr + ": " + valueExpr;
+    }
 }