Implement copy button for ChunkedSnippets component

[huijing]

Context

Even though Starlight has added expressive code, and all our mdx code blocks written with fences has the copy button out-of-the-box, our ChunkedSnippet (and likely FullSnippet) components are not MDX, and therefore we need to roll our own. ¯\_(ツ)_/¯