Add option to skip optimization with wasm-opt

[sisou]

I am currently running wasm-pack three times for our project. Once for the main-thread wasm file, once for the web-worker wasm file, and once to generate types that include both of those builds.

This --no-opt option would help me immensely by reducing our build time by ~30% by not having to wait for the wasm-opt step for the type generation. I cannot disable wasm-opt generally in Cargo.toml, because I still want it for the other two actual build steps.

An alternative to this PR would be a --types-only option, which only outputs a .d.ts file.

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✨✨ 😄 Thanks so much for contributing to wasm-pack! 😄 ✨✨

[drager]

Thank you!