Codespaces can't find SampleRefs.props

[michael-hawker]

#82 broke running the all-samples in Codespaces as there's no SampleRefs.props file by default anymore.

Originally posted by @michael-hawker in #88 (review)

We should figure out a strategy here (also not sure if we want to have configs for each experiment listed or not?).

Also, we may want to guard Codespaces with running a CI job too.

[Arlodotexe]

We'll need to create a prebuild task, perhaps called "Discover Samples" that generates this file.

While we're doing this, we should look at making this task dual-purpose to populate this dropdown in Codespaces with the discovered samples in the repo.