Before 4f37f8643, we were using `innerText` to retrieved the code content for
copying. This preserved the text layout (including newlines), but suffered from
other issues (browser support, performance). With 4f37f8643 we switched to
`textContent`, which works well except in the following case:
When `prettify` formats the code to have line numbers, it removes the newlines
and uses `<li>` elements instead. This affects `textContent`.
This commit fixes this by keeping a reference of the code as text and using that
for copying.
Fixes#17659