KeyBinding without a key identifier (only a modifier key)

Question

What’s the best way to listen for the pressing of a modifier key (like alt) and then run a ViewPlugin in response to that event?

What am I really trying to do?

Enable the clicking of markdown links by wrapping them in an anchor tag (with contenteditable="false" (because contenteditable links don’t work) when the user has the Alt key pressed down.

There’s an extension that almost does this with a link icon, but I want to be able to click the actual link itself.