Page Builder in Drupal

Yes, please see #3251877-16: Add composer.json and use composer to install all the required libraries and perform a clean up. → about the Mercury Editor.

For those looking for other options for page/layout builders in Drupal, you may want to watch this video from OSTraining that lists a few: Exploring Page Builders in Drupal 9

Also you may want to stay tuned for more exciting news: https://dri.es/evolving-drupal-layout-builder-to-an-experience-builder