[Feature Request] Add button to copy code block content

[duskmoon314]

It would be nice to have a button to copy a code block's content like mdBook.

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In addition, we can copy content by selecting and using the browser's hotkeys. However, this looks weird, possibly due to many empty elements. There are many span with class typst-content-fallback but have no child elements.

[Myriad-Dreamin]

I have no idea to identify the raw blocks in a compiled document. Perhaps we can enhance it when typst provides an IR (the IR for HTML export) containing full information for us.

[Myriad-Dreamin]

Since we have customized label now, I believe we can experiment it a bit.

Context, there is a "data-typst-label" to utilize:

https://github.com/typst/typst/blob/bb38a01d0625d5bdab0af50daf479e60c774d90e/crates/typst-svg/src/lib.rs#L254-L256C10