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).

tests/Slider.test.svelte

@@ -14,6 +14,9 @@
   on:change="{(e) => {
     console.log(e.detail); // number
   }}"
+  on:input="{(e) => {
+    console.log(e.detail); // number
+  }}"
 />
 
 <Slider