Consider using `wasm32-unknown-unknown` as the default cargo build target

[jstarry]

Cargo allows specifying a default target when building: https://doc.rust-lang.org/cargo/reference/config.html

Need to check to make sure cargo-web builds don't get affected.

[kellytk]

At some point wasm32-wasi will be worth considering.

For future reference:

• https://wasi.dev/
• https://github.com/CraneStation/wasi
• https://github.com/CraneStation/rust-wasi
• cdylib for the target wasm32-wasi rust-lang/rust#64187
• rustc: Allow the cdylib crate type with wasm32-wasi rust-lang/rust#64188
• Pure-Rust implementation of WASI's open_parent rust-lang/rust#64434
• Support for wasm32-wasi target? drager/wasm-pack#654
• WASI Libraries WebAssembly/WASI#24