chore(number-input): ensure value is a number

Fast-follow to #1555. `ref.value` remains a string if not converted to a number.

src/NumberInput/NumberInput.svelte

@@ -119,7 +119,7 @@
     } else {
       ref.stepDown();
     }
-    value = ref.value;
+    value = +ref.value;
 
     dispatch("input", value);
     dispatch("change", value);