[Enhanced code blocks] Insert newline at the end of copied multi-line text

[seshrs]

Context

Closes #223. Thanks @awdeorio and @ohjuny, this was a great suggestion 😃

Validation

Visit the preview URL and try copying any of the multiline code blocks (including console blocks). A newline is appended to the copied text.

https://preview.sesh.rs/previews/eecs485staff/primer-spec/234/demo/enhanced-code-blocks.html

[github-actions[bot]]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/234/.

^{(Available until Sun Jan 22 2023.)}