meta: add external PRs to project board (#96)

The `pull_request` hook runs in the context of the proposed changes. That means that for forks, this action won't have access to the required secrets for it to complete. As such, PRs from outside contributors won't work correctly.

The `pull_request_target` hook, however, runs in the context of the target branch, and thus has all the permissions it needs. The [github docs for this hook](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target) also indicate that this is the way to go:

> This event allows your workflow to do things like label or comment on pull requests from forks. Avoid using this event if you need to build or run code from the pull request.

## Interesting note for PRs

It seems that when this change is proposed as a PR, the expected action (_add new item to project board_) does not run. However, this does not affect other new PRs. Also, after merging it works as expected, for both external and internal contributors.

.github/workflows/add-to-project.yml

@@ -4,7 +4,7 @@ on:
   issues:
     types:
       - opened
-  pull_request:
+  pull_request_target:
     types:
       - opened