feat(form-core): add a way to prevent registration of a form-element

[gerjanvangeest]

What I did

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[tlouisse]

Ideally the developer using composition inside a FormControl should not be bothered with this.
I think we can do it in such a way that we align this behavior with how modelValues are propagated (LionInputTelDropdown is already considered an "endpoint". (Fields are endpoints, FormGroups aren't))

Fields have this._repropagationRole = 'child'. How this is currently handled for the modelValue repropagation: https://github.com/ing-bank/lion/blob/master/packages/ui/components/input-tel-dropdown/src/LionInputTelDropdown.js#L445

We should solve both (modelValue repropagation and registration)in a more generic way.
Maybe we can do:

In FormControlMixin:

/**
 * Whether 
 * @readonly
 */
get _isEndPoint() {
  return this.__isEndpoint || (this._repropagationRole === 'child');
}
  
/**
 * We should only repropagate modelValue events if we are a group (not if we are an endpoint without children)
 */
_repropagationCondition() {
  return !this._isEndpoint;
}

In FormRegistrarMixin:

// prevent child registration if _isEndpoint

N.B. above is pseude code and there will be a few edge cases we need to tackle as well.
(edge cases will mainly be the situations with options (child) registering to choice-group.)