tests: trigger rextendr CI when extendr PR is merged

We removed interlocking testing from extendr (see https://github.com/extendr/extendr/pull/888/files#diff-faff1af3d8ff408964a57b2e475f69a6b7c7b71c9978cccc8f471798caac2c88). However, we still need to ensure that rextendr tests stay up to date with extendr changes.

The best way for us to do this is to trigger tests in rextendr when we make a change to extendr.

We can probably handle it with [repository_dispatch](https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#repository_dispatch) workflow events.

See related: https://github.com/peter-evans/repository-dispatch

Review the repository's existing GitHub Actions workflows and the repository_dispatch event documentation first. Configure the extendr merge flow to trigger rextendr's test workflow, then verify that an extendr change starts the expected rextendr tests.

Written by the indexing model from the issue text.