add copy algorithm tooltip

[devsnek]

Add a tooltip which copies a text version of the algorithm into the clipboard.

[bakkot]

There's a couple cases this doesn't handle all that well:

• algorithms with nested unordered lists, as in Number::toString
• syntax-direct operations (it just copies the first algorithm block)

[bakkot]

Closing in preference for #290. We could maybe revisit this if we decide that solution isn't adequate, after some time with it.