Add comment-pipe shortcuts to IDE editor modes

[cscheid]

(cf. quarto-dev/quarto-cli#528)

IDE modes should have a shortcut that adds a comment-pipe to a code cell. In general, the IDE will have to know about comment syntax for a number of different languages, but I think it should be possible for us to do the right thing in RStudio, in the VS Code extension, and in emacs.

In general, we'd want to add a custom API entry point to our (future) LSP.

[eteitelbaum]

I would like to endorse this idea. Typing "shift #" and then "shift |" gets cumbersome. It would be nice to have a hash pipe (#|) shortcut!