Feature request: Toolchain selector syntax (`forge +nightly test`)

It would be great to have symlinks to a wrapper that enables rustup-style toolchain selection:

```bash
forge +nightly test
forge +stable test
cast +1.5.0 call ...
```

This would allow quickly switching between toolchains for a single command without needing to run `foundryup -v <version>` first.

Similar to how rustup handles `cargo +nightly build`.