Preserve line breaks on copy command

I have tested successfully on the latest version of Chrome, Safari and
Firefox. I also tested successfully on an Android and iOS devices.

I couldn't tested more exhaustively on other desktop/mobile browser
versions because SauceLabs didn't allowed me to retrieve the pasted
text.

Closes https://github.com/hypothesis/product-backlog/issues/1180

src/sidebar/util/copy-to-clipboard.js

@@ -10,7 +10,7 @@
  * @param {string} text
  */
 export function copyText(text) {
-  const temp = document.createElement('input');
+  const temp = document.createElement('textarea'); // use textarea instead of input to preserve line breaks
   temp.value = text;
   temp.setAttribute('data-testid', 'copy-text');
   // Recipe from https://stackoverflow.com/a/34046084/14463679