Add various copy buttons to code snippets

[NullVoxPopuli]

Kinda like what GitHub implemented, except:

• copy code block as text implemented
• copy code block as image
• copy rendered document as image

what's tricky about this is that the content is all dynamic, so we can't statically render copy buttons with a menu or popover.

• this could be rendered elsewhere in the document, and hidden
• when a pre tag or rendered document is hovered, we could show the elsewhere'd content and change the position to be near the hovered element

[NullVoxPopuli]

use: https://www.npmjs.com/package/html-to-image