Add toolchain hint heuristics on `spin new`

Each language has its own toolchain setup requirements. Ideally we could handle these seamlessly for developers, but until we can figure that out we should try to give more guidance to reduce frustration.

The most obvious example is skipping rustup target add wasm32-wasi, which while documented in the quickstart guide is easy to miss/forget. It would be great if we could detect this situation and print a helpful message at one or more of these points:

• spin new - look for a Cargo.toml in the template?
• spin build - on failure, look for cargo.*wasm32-wasi in a component.build.command?
• spin up - on failure, look for target/wasm32-wasi in the source

In any of these cases we could use the existing check from build.rs: https://github.com/fermyon/spin/blob/60976e41231820ba6b3c4d6e0391814b459c7628/build.rs#L84-L94

Start with the existing toolchain check in build.rs and compare it with the quickstart guide's rustup target add wasm32-wasi requirement. Then inspect the proposed spin new, spin build, and spin up points and determine where the missing-target hint should appear; done means the relevant failure or template case produces helpful guidance.