feat: Automatically install wasm32-unknown-unknown

[saulecabrera]

Implements #9

This commit enables automatic installation of the wasm32-unknown-unknown target for the
build and check commands.

The installation will be performed if:

1. No target param is specified
2. The wasm32-unknown-unknown target is not already installed
3. rustup is available as the toolchain installer

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This implementation is based on cargo-wasi's install_wasi_target, with the difference that I'm not introducing a cache for the target check.

I opted to leave the cache implementation up for discussion mainly because: (i) the cache can potentially introduce false positives if for example, the user removes the target manually after running build or check for the first time, so if we go ahead with the implementation we might need to think about an "eviction" mechanism analogous to cargo wasi self clean or think of a different caching approach (ii) cargo-wasi has several use-cases for the cache: one being the target installation and the other one being update-checks, perhaps if update checks is something that we want to implement we could add the caching at that time? Right now we'd mostly be saving the rustc --print sysroot invocation. Thoughts?

[peterhuene]

This looks good 👍.

Regarding caching the results of the check, I think we can live without that for now but perhaps file an issue to track potentially implementing it if it proves that always having the additional rustc invocation is undesirable (can be avoided by manually specifying the target for now, at least).