DEV: adds an integer validation rule to form-kit (#27985)

Usage:

```
@validation="integer"
```

This commit also adds a default for rules. By default a rule will now be `ruleName: {}`, this avoids all the boilerplate in validation-parser.js.

config/locales/client.en.yml

@@ -2105,6 +2105,7 @@ en:
       errors:
         required: "Required"
         invalid_url: "Must be a valid URL"
+        not_an_integer: "Must be an integer"
         not_accepted: "Must be accepted"
         not_a_number: "Must be a number"
         too_high: "Must be at most %{count}"