Is it possible to write an action that triggers when a reaction is added to an issue or to an issue comment?
You can ask GitHub nicely – https://support.github.com/contact/feedback
I’m waiting for this feature myself…
There is no github event on issue comment reaction. As @jsoref mentioned, you can raise a feature request ticket here: https://support.github.com/contact/feedback?contact[category]=actions .
As an alternative, you can use other event, for instance, edit issue comment (on: issue_comment), labels(on: issues type: labeled)…etc. Please refer to the event doc for more details.