How can I print the name of every template being rendered in HTML comments, when in devMode? Is there a plugin or setting that prints the name of every template being rendered in HTML comments? I noticed this type of feature in a Drupal project and it was useful for debugging.

This is useful if you add it to every template you make, but I'd like to be able to turn it on/off globally for all templates in dev mode:
How can I make a template output its own filename?
So this sounded like a fun problem to solve... so I wrote the Template Comments plugin that does what you want:

This free plugin adds a HTML comment to demarcate {% block %}s and each Twig template that is included or extended.