Externalize clipboard copying to the
[clippie](https://github.com/silverwind/clippie) module which I feel I
can maintain outside this repo for shared benefit with my other
projects.
The module is feature-equivalent to the previous code and has one
improvement where it sets `aria-hidden` on the fallback textarea,
preventing screen readers from picking it up. Also it support `Array` of
`content` as well to copy multiple items at once, in case it's ever
needed.
