[Request] use render_callback in development mode

[braco]

Based on this suggestion:

WordPress/gutenberg#10444 (comment)

It might be better to use render_callback while in development mode to avoid the dreaded "external source" errors

[chrisvanpatten]

Just popping in to make clear that my comment was not an official recommendation of the Gutenberg project, just based on my own personal experience working with Gutenberg for client projects 😊 I think it's a helpful recommendation, but I wouldn't take too much stock in it!