feat: Client-side CustomEvent for the form element

[ayoayco]

We can use CustomEvent to allow users to attach callbacks when validation is triggered

[Icyscools]

Is it a same as #268?

[ayoayco]

@Icyscools Ah nope but I see they do look similar. I will clarify the descriptions later. This one is for allowing users to subscribe to validation events (when form becomes invalid) the other is for users to give their own custom logic for validation.