Enable `--all` to uninstall all managed Pythons (#4932)
## Summary
Allows `--all` as an alternative to specifying specific targets.
## Test Plan
Verified that `cargo run python uninstall` still fails.
```
❯ cargo run python uninstall --all
Compiling uv-cli v0.0.1 (/Users/crmarsh/workspace/puffin/crates/uv-cli)
Compiling uv v0.2.23 (/Users/crmarsh/workspace/puffin/crates/uv)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 2.86s
Running `target/debug/uv python uninstall --all`
warning: `uv python uninstall` is experimental and may change without warning.
Searching for Python installations
Found existing installation: cpython-3.9.18-macos-aarch64-none
Found existing installation: cpython-3.8.18-macos-aarch64-none
Found existing installation: cpython-3.8.12-macos-aarch64-none
Found existing installation: cpython-3.12.1-macos-aarch64-none
Found existing installation: cpython-3.11.7-macos-aarch64-none
Found existing installation: cpython-3.10.13-macos-aarch64-none
Uninstalled cpython-3.10.13-macos-aarch64-none
Uninstalled cpython-3.11.7-macos-aarch64-none
Uninstalled cpython-3.12.1-macos-aarch64-none
Uninstalled cpython-3.8.12-macos-aarch64-none
Uninstalled cpython-3.8.18-macos-aarch64-none
Uninstalled cpython-3.9.18-macos-aarch64-none
Uninstalled 6 versions in 479ms
```
