Add support for development dependencies

[charliermarsh]

Summary

Externally, development dependencies are currently structured as a flat list of PEP 580-compatible requirements:

[tool.uv]
dev-dependencies = ["werkzeug"]

When locking, we lock all development dependencies; when syncing, users can provide --dev.

Internally, though, we model them as dependency groups, similar to Poetry, PDM, and PEP 735. This enables us to change out the user-facing frontend without changing the internal implementation, once we've decided how these should be exposed to users.

A few important decisions encoded in the implementation (which we can change later):

1. Groups are enabled globally, for all dependencies. This differs from extras, which are enabled on a per-requirement basis. Note, however, that we'll only discover groups for uv-enabled packages anyway.
2. Installing a group requires installing the base package. We rely on this in PubGrub to ensure that we resolve to the same version (even though we only expect groups to come from workspace dependencies anyway, which are unique). But anyway, that's encoded in the resolver right now, just as it is for extras.

[charliermarsh]

I find the terminology here really confusing -- that we have both extras and dependency groups -- but, what can we do? I'm honestly considering using dev_dependencies everywhere internally instead of dependency groups.

[charliermarsh]

I decided to rework the terminology (internally) to center on development dependencies (even though it still supports groups internally), since that's the only thing they're actually used for right now.