Add test discovery and runnables for R files

[aymennasri]

Similar to Python's solution in #12080, testthat is the standard package for it.

Would be amazing to have more support for R as more and more people are coming to Zed after the REPL feature.

[SomeoneToIgnore]

Zed core has nothing related to R, it is only supported in extensions.
https://github.com/ocsmit/zed-r/ is the current one, so this feature request has to go there.