[DataModel] Expose value changes as a DOM event to react with Action

[titouanmathis]

This can be useful to trigger other behaviors when working with forms. Another solution would be to emit a custom event on the DataBind component when the value changes, allowing us to react to it with the Action component.

<input type="text" 
  data-component="DataModel Action"
  data-option-on:custom-input="console.log('input has changed')"
  value="...">

[titouanmathis]

Reframing this issue to reflect what is now available.

The original motivation — reacting to a value change — is partly covered by the DataEffect component, which runs a data-option-effect expression whenever the bound value changes.

What is still missing is a real DOM event emitted on change, so a change can be handled declaratively through Action's data-on:* syntax (as in the original example). Keeping this open, scoped to that: emit a custom DOM event from DataModel (and/or DataBind) when the value changes.