Ensure that selection sliders return integer values for indices.

According to the nouislider docs, it returns values to 2 decimal places, and the place to make sure we have integers is in the to transformation function.

Fixes #3363

Author: jasongrout

packages/controls/src/widget_selection.ts

@@ -765,7 +765,7 @@ export class SelectionSliderView extends DescriptionView {
       direction: orientation === 'horizontal' ? 'ltr' : 'rtl',
       format: {
         from: (value: string): number => Number(value),
-        to: (value: number): number => value,
+        to: (value: number): number => Math.round(value),
       },
     });