Add ability to add caption to a code block

Feature request from https://wordpress.org/support/topic/feature-request-ability-to-add-a-caption/

Add the ability to label/caption individual code blocks on the page, similar to image caption in Image & Gallery blocks.

Start by locating the plugin's code block and Gutenberg block implementation, then trace how individual blocks are rendered and configured. Define how a caption is entered and displayed in relation to the existing Image and Gallery block behavior; done means each code block can have an optional visible caption without disrupting syntax highlighting.

Written by the indexing model from the issue text.