Add markdown reporter

[Bisaloo]

It would be very nice to have a markdown reporter, so that test results can be displayed directly using the new GitHub Actions job summaries feature without having to dig into the logs.

If this seems like a desirable feature for testthat, I could try to have a look at implementation but can't guarantee a PR as it might be beyond my abilities.

[hadley]

We also need to consider who will be responsible for turning this reporter on — should testthat automatically use this reporter if GITHUB_STEP_SUMMARY is set, or should r-lib/actions be responsible for explicitly enabling it.

Thoughts:

• Needs to expose skips + warnings
• Can we make clickable links from the srcrefs?

[hadley]

See discussion in #1635: due to GitHub's implementation of this feature, it's not that useful, and it doesn't seem like we can make it more useful on our side.