Add a "copy content" button to code blocks

[abey79]

Having a "copy content" button appearing on hover over code blocks (typically top right) is quite common on modern web template (e.g. in our own docs at Rerun). It would be great to have something similar in egui_commonmark.

This is currently covered by having the code block text selectable, but this feels a bit clunky. (It's nice to have though, in cases the user wants to copy only part of the text.)

Related to:

• Copyable text #10