Exposes the option added in #4416. Adds `--toolchain-preference` and
`tool.uv.toolchain-preference` to configure if system or managed
toolchains are preferred. Users can opt-out of managed toolchains or
system toolchains entirely as well.
## Summary
In a workspace, we now read configuration from the workspace root.
Previously, we read configuration from the first `pyproject.toml` or
`uv.toml` file in path -- but in a workspace, that would often be the
_project_ rather than the workspace configuration.
We need to read configuration from the workspace root, rather than its
members, because we lock the workspace globally, so all configuration
applies to the workspace globally.
As part of this change, the `uv-workspace` crate has been renamed to
`uv-settings` and its purpose has been narrowed significantly (it no
longer discovers a workspace; instead, it just reads the settings from a
directory).
If a user has a `uv.toml` in their directory or in a parent directory
but is _not_ in a workspace, we will still respect that use-case as
before.
Closes#4249.
Charlie Marsh