fix(slider): dispatch on:input event (#1906)

Fixes #1643

The dispatched `on:change` event in `Slider` only fires when the slider thumb is released. `on:input` fires when actively dragging the thumb, and should fire on every increment (e.g., 1, 2, 3).

src/Slider/Slider.svelte

@@ -1,6 +1,7 @@
 <script>
   /**
    * @event {number} change
+   * @event {number} input
    */
 
   /** Specify the value of the slider */
@@ -109,6 +110,7 @@
     }
 
     value = nextValue;
+    dispatch("input", value);
   }
 
   $: labelId = `label-${id}`;