We need to get back the "Copy to clipboard" button on Lean code snippets

[lovettchris]

Description

Copy/paste the output of Alectryon on lean code snippets produces a big mess in your VS code text editor.

Detailed behaviour

Go to https://leanprover.github.io/lean4/doc/monads/functors.lean.html and select the first lean snippet for List.map and paste it into VS code and you will get this:

#eval
["1", "2", "3"]

List.map (λ
x =>
toString
x) [
1,
2,
3] -- ["1", "2", "3"]