Code copy button

### What

Add a button that allows you to *Copy* code from code block while in preview mode.

### Why

Allows a user to easily copy code snippets from the preview mode.
### How

[GitHub example of Obsidian plugin](https://github.com/jdbrice/obsidian-code-block-copy)

Start by tracing the preview-mode rendering for code blocks in the Simplenote Electron application; the issue does not name a file, test, or entry point. The work is done when code blocks in preview mode expose a Copy button and the button copies the snippet for the user.

Written by the indexing model from the issue text.