Add `--extra` to `uv add` and enable fine grained updates (#4566)
## Summary
- Adds a `--extra` flag to `uv add` that allows activating extras
without the PEP508 syntax.
- `uv add` now errors if the update is ambiguous (e.g. the dependency is
present twice with different markers)
- `uv add` is smarter about updates. For example, `uv add flask==3.0.0`
followed by `uv add flask --extra dotenv` preserves the previous version
specifier.
Resolves https://github.com/astral-sh/uv/issues/4419.
