[WIP] add copy button for lstlistings

[evoludolab]

This PR aims to address #2588.

The lstlisting.js adds a hook to replace all download links in HTML documents by a download button after the document has been loaded.

Following @dginev's encouragement, I polished my JavaScript and created my own icons to avoid any copyright issues. All SVG icons are in the CSS in order to minimize the number of files, while maintaining customizability.

I marked the PR as a draft because I'm struggling with

• where to make changes or place the files,
• how to best name things,
• how to activate this JS extension during post-processing.

Currently, I pass a JS file to LaTeXML with --javascript=<custom.js>, which includes the code from lstlisting.js and the resulting buttons work as expected.

I'm looking forward to your feedback/guidance on this - or watch what changes you make to the code 😄.