uv-resolver: introduce new UniversalMarker type
This effectively combines a PEP 508 marker and an as-yet-specified
marker for expressing conflicts among extras and groups.
This just defines the type and threads it through most of the various
points in the code that previously used `MarkerTree` only. Some parts
do still continue to use `MarkerTree` specifically, e.g., when dealing
with non-universal resolution or exporting to `requirements.txt`.
This doesn't change any behavior.
