Plugin does not work when the Classic Editor Plugin is enabled

[yonkov]

Problem: The plugin is currently not compatible with the classic WordPress editor. A possible workaround would be to display a notice for users who are still using the Classic Editor to switch to the Gutenberg Editor in order to use the plugin.

** Useful links:**

1. https://www.wpbeginner.com/wp-tutorials/how-to-add-admin-notices-in-wordpress/
2. How to detect "is-gutenberg" via PHP and JS WordPress/gutenberg#12200

Success criteria: A dismissable php notice on top of the post editor would suffice.