Add CI test environment with no extras installed

[bjchambers]

Specifically, we need to verify that with only the "core" dependencies the "core" functionality works. To do this, we may need to mark tests that are (or are not) part of the core functionality so they can be skipped.

[epinzur]

#113 adds support marking tests as an "extra", and then won't be tested unless --runextras is passed to pytest.

See: https://github.com/datastax/graph-rag/pull/113/files#diff-36a7b54d2155f4400162aa7ba9091976f9f45e6539b1d31671c3e6272694f80aR8 for an example of this.