Add `uv toolchain list` (#4163)
Adds the `uv toolchain` namespace and a `list` command to get us
started.
```
❯ cargo run -q -- toolchain list
warning: `uv toolchain list` is experimental and may change without warning.
3.8.12 (cpython-3.8.12-macos-aarch64-none)
3.8.13 (cpython-3.8.13-macos-aarch64-none)
3.8.14 (cpython-3.8.14-macos-aarch64-none)
3.8.15 (cpython-3.8.15-macos-aarch64-none)
3.8.16 (cpython-3.8.16-macos-aarch64-none)
3.8.17 (cpython-3.8.17-macos-aarch64-none)
3.8.18 (cpython-3.8.18-macos-aarch64-none)
3.8.18 (cpython-3.8.18-macos-aarch64-none)
3.8.19 (cpython-3.8.19-macos-aarch64-none)
3.9.2 (cpython-3.9.2-macos-aarch64-none)
3.9.3 (cpython-3.9.3-macos-aarch64-none)
3.9.4 (cpython-3.9.4-macos-aarch64-none)
3.9.5 (cpython-3.9.5-macos-aarch64-none)
3.9.6 (cpython-3.9.6-macos-aarch64-none)
3.9.7 (cpython-3.9.7-macos-aarch64-none)
3.9.10 (cpython-3.9.10-macos-aarch64-none)
3.9.11 (cpython-3.9.11-macos-aarch64-none)
3.9.12 (cpython-3.9.12-macos-aarch64-none)
3.9.13 (cpython-3.9.13-macos-aarch64-none)
3.9.14 (cpython-3.9.14-macos-aarch64-none)
3.9.15 (cpython-3.9.15-macos-aarch64-none)
3.9.16 (cpython-3.9.16-macos-aarch64-none)
3.9.17 (cpython-3.9.17-macos-aarch64-none)
3.9.18 (cpython-3.9.18-macos-aarch64-none)
3.9.19 (cpython-3.9.19-macos-aarch64-none)
3.10.0 (cpython-3.10.0-macos-aarch64-none)
3.10.2 (cpython-3.10.2-macos-aarch64-none)
3.10.3 (cpython-3.10.3-macos-aarch64-none)
3.10.4 (cpython-3.10.4-macos-aarch64-none)
3.10.5 (cpython-3.10.5-macos-aarch64-none)
3.10.6 (cpython-3.10.6-macos-aarch64-none)
3.10.7 (cpython-3.10.7-macos-aarch64-none)
3.10.8 (cpython-3.10.8-macos-aarch64-none)
3.10.9 (cpython-3.10.9-macos-aarch64-none)
3.10.11 (cpython-3.10.11-macos-aarch64-none)
3.10.12 (cpython-3.10.12-macos-aarch64-none)
3.10.13 (cpython-3.10.13-macos-aarch64-none)
3.10.14 (cpython-3.10.14-macos-aarch64-none)
3.11.1 (cpython-3.11.1-macos-aarch64-none)
3.11.3 (cpython-3.11.3-macos-aarch64-none)
3.11.4 (cpython-3.11.4-macos-aarch64-none)
3.11.5 (cpython-3.11.5-macos-aarch64-none)
3.11.6 (cpython-3.11.6-macos-aarch64-none)
3.11.7 (cpython-3.11.7-macos-aarch64-none)
3.11.8 (cpython-3.11.8-macos-aarch64-none)
3.11.9 (cpython-3.11.9-macos-aarch64-none)
3.12.0 (cpython-3.12.0-macos-aarch64-none)
3.12.1 (cpython-3.12.1-macos-aarch64-none)
3.12.2 (cpython-3.12.2-macos-aarch64-none)
3.12.3 (cpython-3.12.3-macos-aarch64-none)
```
Closes https://github.com/astral-sh/uv/issues/4189
