Add copy & expand buttons to syntax-highlighted code blocks

[ryelle]

Fixes #2572 — This adds the copy & expand buttons from Developer to the code blocks on Learn. I started from the function-reference.js script, but removed the jQuery dependency & other developer-custom code.

This "block" is not actually a block, it's just using the folder structure for the build process. On init, it registers a script and style, and when the code block is rendered, the script and style are loaded. When the JS sees a code block, if it's using the syntax highlighting, it injects a copy button & maybe the expand button.

Initially I started this as a block that could live in wporg-mu-plugins (and it worked), but trying to get it to work with the existing developer blocks might be a bit more coordination than we want to do for Learn's launch. Moving this back into mu-plugins could be a future improvement though.

Before	After

Note: I restricted this to syntax-highlighted only because these blocks also have some styles that make it look better (padding, primarily). For example, if it's on all code blocks, these look awkward:

To test

• View a lesson with code syntax blocks, ex https://learn.wordpress.org/lesson/accessing-wordpress-data/
• The code blocks should have copy buttons, and if they're longer than 12 lines, they should be collapsed with an expand button
• View a lesson with code blocks without syntax highlighting, ex http://localhost:8888/lesson/javascript-concepts/
• No copy or expand buttons

[adamwoodnz]

Works for me, very nice!