[Feature Request] Add `copy-tex` to Katex

[julien-blanchon]

💭 Describe the feature

The copy-tex script enable copy in latex (instead of ugly unicode like).

💡 Proposed Solution

Add <script src="https://cdn.jsdelivr.net/npm/katex@0.16.4/dist/contrib/copy-tex.min.js" integrity="sha384-ww/583aHhxWkz5DEVn6OKtNiIaLi2iBRNZXfJRiY1Ai7tnJ9UXpEsyvOITVpTl4A" crossorigin="anonymous"></script> to the header.

See https://github.com/KaTeX/KaTeX/tree/main/contrib/copy-tex