Copy to clipboard: Removed ClipboardJS dependency

[RunDevelopment]

This removes Copy to clipboard's dependency on ClipboardJS.

ClipboardJS has been replaced with a few functions that behave similarly. The actual copying is implemented by the Async + fallback combo here. This alone should give us very good browser compatibility (works in IE11).

Older FF versions do not support copying text to the clipboard at all. For these browsers, we use the same trick as ClipboardJS: we just select the text to copy and ask the user to press Ctrl+C. It's implemented slightly differently, however. ClipboardJS creates a temporary invisible element and selects the text of that but we can just select the text of the <code> element we want to copy (makes everything a lot easier).
This may scroll the code block but I didn't give this much consideration since it will only affect a very small portion of our users.

Prism now has 0 required (runtime) dependencies on other libraries.

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This resolves #2783
This resolves #2565
This resolves #2286

[github-actions]

JS File Size Changes (gzipped)

A total of 1 files have changed, with a combined diff of +38 B (+6.0%).

file	master	pull	size diff	% diff
plugins/copy-to-clipboard/prism-copy-to-clipboard.min.js	632 B	670 B	+38 B	+6.0%

Generated by 🚫 dangerJS against a89058b

[RunDevelopment]

The +38B in bundle size should be viewed in the context that we don't have to download the 3kB of ClipboardJS anymore.