build: leave comment if command is not recognized

Author: Planeshifter

.github/workflows/slash_commands.yml

@@ -56,6 +56,12 @@ jobs:
               -H "Authorization: Bearer ${{ secrets.STDLIB_BOT_GITHUB_TOKEN }}" \
               "https://api.github.com/repos/${{ github.repository }}/issues/comments/${{ github.event.comment.id }}/reactions" \
               -d '{"content":"eyes"}'
+          else
+            curl -X POST \
+              -H "Accept: application/vnd.github.v3+json" \
+              -H "Authorization: Bearer ${{ secrets.STDLIB_BOT_GITHUB_TOKEN }}" \
+              "https://api.github.com/repos/${{ github.repository }}/issues/${{ github.event.issue.number }}/comments" \
+              -d '{"body":"@${{ github.event.comment.user.login }}, slash command not recognized. Please use `/stdlib help` to view available commands."}'
           fi
 
   # Define a job for checking for required files: