Expand the conditionals UI to support editing and updating complex nested conditionals

The \Drupal\webform\WebformSubmissionConditionsValidator would also have to be updated to support nested conditions. This is a major undertaking which I personally don't have the resources or mental capacity to pursue.

We might consider copying the Views UI but I find Views nested filters confusing. The Conditional Fields module UI is using multi-step process and it is a little overwhelming.

I have to give @quicksketch credit for building a reasonable UI for complex use cases in the Webform module for D7.

I have a hard time wrapping my head around processing nested conditions and then building a sensible UI is even more challenging. A lot of form builders on the market have avoided supporting nested conditions.

Finally, I think Drupal core might need to update #states API and provide some sort of server-side validation to admin forms, especially if Form API starts supporting React base form.

A very reasonable MVP would be to update WebformSubmissionConditionsValidator to support nested conditions and continue to use YAML for writing advanced #states. There should be enough existing test coverage to prevent regressions while improving WebformSubmissionConditionsValidator.

To begin to wrap my head around this issue I just wanted to post some YAML that illustrates a nested #states conditional (via a_and_b_or_c)

a:
  '#type': checkbox
  '#title': a
b:
  '#type': checkbox
  '#title': b
c:
  '#type': checkbox
  '#title': c
a_and_b:
  '#type': textfield
  '#title': '[a and b] = required'
  '#states':
    required:
      ':input[name="a"]':
        checked: true
      ':input[name="b"]':
        checked: true
a_and_b_or_c:
  '#type': textfield
  '#title': '[a and b] or c = required'
  '#states':
    required:
      -
        - ':input[name="a"]':
            checked: true
          ':input[name="b"]':
            checked: true
      - or
      - ':input[name="c"]':
          checked: true

I really don't want to support nested conditionals via the UI because the UX becomes too complex. For an element's #states advanced users can edit the YAML source. The only problem is that a handler's conditional logic's YAML can't be edited.

I am tempted to add an Ajax button link to switch on the ability to edit a #states YAML source.

The attached patch adds an 'Edit source' button to the 'Conditional logic' builder. I will follow up with a change record.

Here is the change record.

Some minor cleanup.

I committed the patch. Please download the latest dev release to review.