chore: add previewMode to middleware demo

Author: pieh

demos/middleware/pages/api/enterPreview.js

@@ -0,0 +1,6 @@
+export default async function preview(req, res) {
+  // Enable Preview Mode by setting the cookies
+  res.setPreviewData({})
+
+  res.status(200).json({ name: 'preview mode' })
+}

demos/middleware/pages/api/exitPreview.js

@@ -0,0 +1,8 @@
+export default async function exit(_, res) {
+  // Exit the current user from "Preview Mode". This function accepts no args.
+  res.clearPreviewData()
+
+  // Redirect the user back to the index page.
+  res.writeHead(307, { Location: '/' })
+  res.end()
+}