Standardize UI widget extentions

[dafeder]

We have a WidgetRouter in JSON Form Widget that has gotten a bit unwieldy. It is not exactly a router it is also a collection of methods that all transform default field types into custom ones based on UI schema. This would be better implemented if each field type/widget/whatever the right word would be was its own class, following am interface. The most obvious pattern would be to implement these as plugins. So for instance, WidgetRouter::handleTextAreaElement(), currently like this:

  /**
   * Helper function for getting a textarea element.
   *
   * @param mixed $spec
   *   Object with spec for UI options.
   * @param array $element
   *   Element to convert into textarea.
   *
   * @return array
   *   The element configured as textarea.
   */
  public function handleTextareaElement(mixed $spec, array $element) {
    $element['#type'] = 'textarea';
    unset($element['#maxlength']);
    if (isset($spec->textFormat)) {
      $element['#type'] = 'text_format';
      $element['#format'] = $spec->textFormat;
      $element['#allowed_formats'] = [
        $spec->textFormat,
      ];
    }
    if (isset($spec->rows)) {
      $element['#rows'] = $spec->rows;
    }
    if (isset($spec->cols)) {
      $element['#cols'] = $spec->cols;
    }
    return $element;
  }

might become a class called TextArea, something like this:

class TextArea implements JsonWidgetInterface {
  /**
   * {@inheritdoc}
   */
  public function applyWidget(mixed $spec, array $element): array {
    ...
  }
}

Not fully thought-through but something like this would make this all cleaner.