create sizer that sets item size with flex-basis

works with the flex-grow we set initially for the sub lists

src/resizer/sizer.js

@@ -97,4 +97,11 @@ class Sizer {
     }
 }
 
-module.exports = {Sizer};
+class FlexSizer extends Sizer {
+    setItemSize(item, size) {
+        item.style.flexGrow = '0';
+        item.style.flexBasis = `${Math.round(size)}px`;
+    }
+}
+
+module.exports = {Sizer, FlexSizer};