feat: `testthat::snapshot_download_gh()` should prompt user to run accept/review code

[schloerke]

I believe there is a lot of context switching when trying to get snapshots downloaded and accepting them.

With IDEs being better about clicking code to run, after all downloads are completed, I'm wanting to be prompted with:

── Snapshots ─────
To review and process snapshots locally:
* Run `testthat::snapshot_accept()` to accept all changes.
* Run `testthat::snapshot_review()` to review all changes.

Currently, this prompt is only within the GHA logs, not where the code is being run.