Emit Checkbox change event after modifying internal state (#1013)

The checkbox's internal state doesn't match the visual checked /
unchecked state. The behaviour is visible when using 1-way binding on
the checkbox and using the on:change event. The internal state is always
the opposite of the visual state.

src/Checkbox/Checkbox.svelte

@@ -89,13 +89,13 @@
       name="{name}"
       readonly="{readonly}"
       class:bx--checkbox="{true}"
-      on:change
       on:change="{() => {
         checked = !checked;
         group = group.includes(value)
           ? group.filter((_value) => _value !== value)
           : [...group, value];
       }}"
+      on:change
       on:blur
     />
     <label for="{id}" title="{title}" class:bx--checkbox-label="{true}">