Adds hidden `--preview` / `--no-preview` flags with `UV_PREVIEW`
environment variable support. Copies the `PreviewMode` type from Ruff.
Does a little bit of extra work to port `uv run` to the new settings
model.
Note we allow `uv run` invocations without preview and only use its
presence to toggle an experimental warning.
## Test plan
```
❯ cargo run -q -- run --no-workspace -- python --version
warning: `uv run` is experimental and may change without warning.
Python 3.12.2
❯ cargo run -q -- run --no-workspace --preview -- python --version
Python 3.12.2
❯ UV_PREVIEW=1 cargo run -q -- run --no-workspace -- python --version
Python 3.12.2
```
