Code examples in the handbooks should expand/collapse

[siobhan]

Some of the code examples in the handbooks are really long. See some of the ones on here for example: ​https://developer.wordpress.org/themes/basics/theme-functions/

If a code example is over a certain number of lines can we make them collapse (like we do in the code reference: ​https://developer.wordpress.org/reference/functions/get_posts/ )

[coffee2code]

In 1429:

developer.wordpress.org: Enable expand/collapse functionality for syntax-highlighted code in handbooks.

• Enqueue function-reference.js on handbook pages as well
• Wrap handbook syntax-highlighted code in similar markup the JS expects as used on coderef pages
• Modify JS to handle multiple expandable code blocks on a page
• Change styling for expand/collapse links since for handbooks only one link will be present in that section

Fixes #959.