[misc] adds script for exporting posts from discourse to github issues

[iris-garden]

[!NOTE] I don't think it necessarily makes sense to merge this into main; just checking it in here for review.

An example of a migrated post with a link to another migrated post in it can be found at https://github.com/iris-garden/test-process/issues/3118. The rest of the open issues in that repo were also migrated using this script. I'd recommend taking a look there rather than trying to run the script as part of the review process, as it took 8 hours to open all the Github Issues, due to rate limiting on the Github API 🙃