Expose an indicator of R and/or Python project-hood for use in when clauses

[jennybc]

If you open an R package in Positron, before it has any tests, and go into the test explorer, you see this:

If the workspace is clearly an R package, I think we'd like to not see the contribution from the positron-python extension here:

https://github.com/posit-dev/positron-python/blob/98da4f51910f36c1d898a96461acdf19b662d19c/package.json#L2028-L2032

but I think that would require some work/coordination in positron that provides the positron R and Python extensions some notion of "this is an R project" or "this is a Python" project, for use in the when clause.

Originally part of #1808 (comment).