In profiling mode, tell wasm-opt to retain debug symbols.

[jamesyoungman]

Make sure these boxes are checked! 📦✅

• [x] You have the latest version of rustfmt installed

$ rustup component add rustfmt

• [x] You ran cargo fmt on the code base before submitting
• [x] You reference which issue is being closed in the PR text

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