Allow disabling CLI dependency downloads

[CathalMullan]

Feature Request

For environments like NixOS, auto-downloading binaries like wasm-bindgen isn't ideal.

Similar issue for the rustup usage for auto-downloading of toolchain targets. rustup may not even be present in certain environments.

Add a way to disable these downloads.

Implement Suggestion

Add a CLI feature to disable downloads, similar to the "no_downloads" feature of cargo-leptos. Just look for the tools in $PATH instead.

Instead of using rustup to check what targets are currently installed, you could use rustc --print sysroot instead.

I have a (messy) example here.