Eagerly validate form input

Is your feature request related to a problem? Please describe

A form can be fed with input, and that input may be broken. Unfortunately we don't eagerly validate such broken input, i.e. aaa for a field that requires C-\d+ pattern validation.

The result is that a user MUST change the field or submit the entire form to learn about this error:

Describe the solution you'd like

• [ ] We should treat invalid external input same as user input and eagerly validate the form.

Describe alternatives you've considered

None.

Additional context

Suggested by @christian-konrad here.

I have been going through the Form implementation. Is this one open for contribution?

This and https://github.com/bpmn-io/form-js/issues/382 need to be considered together.