Use correct Set.add invocation in Checkbox Validator

The [Set.add]() method doesn't accept variable arguments. To replace the
`...` splatting, this commit loops over values and adds them
individually.

[Set.add]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/add

Author: seanpdoyle

CHANGELOG.md

@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+*   Use `Set.add` correctly
+
+    *Sean Doyle*
+
 ## 0.1.1 - 2024-09-27
 
 *   Expand version matrix to include `[email protected]` and `[email protected]`