Support discovering and running tests in doc comments

[JosephTLyons]

Something I greatly miss in Gleam is ability to rust tests in doc comments. Having to manually check that code in comments is compatible with the latest package code is a pretty big pain point. This might be too big of a lift or outside the scope of startest, and startest would likely have to form its own opinions on how these are defined, which probably isn't ideal, but this would certainly add to the likelihood of someone reaching for startest over gleeunit.