Add webform support

I've implemented most of the PR comments.

I think the one regarding the documentation was already covered by ld_form README file.

I added additional type attribute to custom element because that data was lost after I utilized more ld_form code and simplified the output.

I was surprised by isApiResponse method code quadruplication. This should probably be caught before.

New output looks like:

  "content": {
    "element": "drupal-form-webform-submission-to-spar-add-form",
    "formId": "webform_submission_to_spar_add_form",
    "attributes": {
      "class": [
        "webform-submission-form",
        "webform-submission-add-form",
        "webform-submission-to-spar-form",
        "webform-submission-to-spar-add-form",
        "webform-submission-to-spar-add-form",
        "webform-submission-to-spar-form",
        "js-webform-details-toggle",
        "webform-details-toggle"
      ],
      "dataDrupalSelector": "webform-submission-to-spar-add-form"
    },
    "method": "post",
    "type": "webform",
    "title": "to spar",
    "content": "<div class=\"form-type-checkbox js-form-item form-item js-form-type-checkbox form-type--checkbox form-type--boolean js-form-item-going form-item--going\">\n            <input data-drupal-selector=\"edit-going\" type=\"checkbox\" id=\"edit-going\" name=\"going\" value=\"1\" class=\"form-checkbox form-boolean form-boolean--type-checkbox\"/>\n\n              <label for=\"edit-going\" class=\"form-item__label option\">going</label>\n            </div>\n<div class=\"form-type-textfield js-form-item form-item js-form-type-textfield form-type--textfield js-form-item-how-much-to-spend form-item--how-much-to-spend\">\n            <label for=\"edit-how-much-to-spend\" class=\"form-item__label\">how much to spend</label>\n              <input data-drupal-selector=\"edit-how-much-to-spend\" type=\"text\" id=\"edit-how-much-to-spend\" name=\"how_much_to_spend\" value=\"\" size=\"60\" maxlength=\"255\" class=\"form-text form-element form-element--type-text form-element--api-textfield\"/>\n          </div>\n<input data-drupal-selector=\"form-jddxnw6y8r0c23sukubghfixbb61zfxvanzlwlgu5ei\" type=\"hidden\" name=\"form_build_id\" value=\"form-Jddxnw6Y8r0C23sUKubGHFixBb61zFxvANZLWlGU5eI\"/><input data-drupal-selector=\"edit-webform-submission-to-spar-add-form-form-token\" type=\"hidden\" name=\"form_token\" value=\"h7FrU478ituV4WXR32dKPgTnxogEMsZMbxglbvt_Ewo\"/><input data-drupal-selector=\"edit-webform-submission-to-spar-add-form\" type=\"hidden\" name=\"form_id\" value=\"webform_submission_to_spar_add_form\"/><div data-drupal-selector=\"edit-actions\" class=\"form-actions js-form-wrapper form-wrapper\" id=\"edit-actions\"><input class=\"webform-button--submit button button--primary js-form-submit form-submit\" data-drupal-selector=\"edit-submit\" type=\"submit\" id=\"edit-submit\" name=\"op\" value=\"Submit\"/>\n</div>\n\n"
  },

Enabling confirmation was not problematic but there's a strange access check in webform that allowed me to access confirmation page only when logged in as admin:

/webform/src/WebformEntityAccessControlHandler::checkAccess

    // Check 'view' operation use 'submission_create' when viewing rendered
    // HTML webform or use access 'configuration' when requesting a
    // webform's configuration via REST or JSON API.
    // @see https://www.drupal.org/project/webform/issues/2956771
    if ($operation === 'view') {
      // Check is current request if for HTML.
      $is_html = ($this->requestStack->getCurrentRequest()->getRequestFormat() === 'html');
      // Make sure JSON API 1.x requests format which is 'html' is
      // detected properly.
      // @see https://www.drupal.org/project/jsonapi/issues/2877584
      $is_jsonapi = (strpos($this->requestStack->getCurrentRequest()->getPathInfo(), '/jsonapi/') === 0) ? TRUE : FALSE;
      if ($is_html && !$is_jsonapi) {
        $access_result = $this->accessRulesManager->checkWebformAccess('create', $account, $entity);
      }
      else {
        if ($account->hasPermission('access any webform configuration') || ($account->hasPermission('access own webform configuration') && $is_owner)) {
          $access_result = WebformAccessResult::allowed($entity, TRUE);
        }
        else {
          $access_result = $this->accessRulesManager->checkWebformAccess('configuration', $account, $entity); // <- this denies access to confirmation page !!!
        }
      }
      if ($access_result instanceof AccessResultReasonInterface) {
        $access_result->setReason('Access to webform configuration is required.');
      }
      return $access_result->addCacheContexts(['url.path', 'request_format']);
    }

Ok, I figured it out and added a section to the README.md file.

Merged.
Bug hunting season starts now.