Add slide mode with reveal.js

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

cypress/support/get-iframe-content.ts

@@ -0,0 +1,32 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+declare namespace Cypress {
+  interface Chainable {
+    getIframeBody(): Chainable<Element>
+    getReveal(): Chainable<Element>
+    getMarkdownBody(): Chainable<Element>
+  }
+}
+
+Cypress.Commands.add('getIframeBody', () => {
+  return cy
+    .get(`iframe[data-cy="documentIframe"][data-content-ready="true"]`)
+    .should('be.visible')
+    .its('0.contentDocument')
+    .should('exist')
+    .its('body')
+    .should('not.be.undefined')
+    .then(cy.wrap.bind(cy))
+})
+
+Cypress.Commands.add('getReveal', () => {
+  return cy.getIframeBody().find('.reveal')
+})
+
+Cypress.Commands.add('getMarkdownBody', () => {
+  return cy.getIframeBody().find('.markdown-body')
+})