Add manual events as "Spell Check" workflow triggers

The `workflow_dispatch` event allows triggering the workflow via the GitHub web interface. This makes it easy to trigger
an immediate workflow run after some relevant external change.

The `repository_dispatch` event allows triggering workflows via the GitHub API. This might be useful for triggering an
immediate check in multiple relevant repositories after an external change, or some automated process. Although we don't
have any specific need for this event at the moment, the event has no impact on the workflow, so there is no reason
against having it. It is the sort of thing that can end up being useful if it is already in consistently in place, but
not worth setting up on demand, since the effort to set it up is greater than the effort to trigger all the workflows
manually.

Author: per1234

.github/workflows/spell-check-task.yml

@@ -1,11 +1,14 @@
 name: Spell Check
 
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
 on:
   push:
   pull_request:
   schedule:
     # Run every Tuesday at 8 AM UTC to catch new misspelling detections resulting from dictionary updates.
     - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
 
 jobs:
   spellcheck: