Replace cargo-generate with `wasm-pack new --template`

[jonknapp]

It appears that wasm-pack's new command can start a project from an existing template and we no longer need to depend on "cargo-generate" as a setup step in the book. If they are functionally equivalent, updating the docs can reduce setup time, complexity, and reliance on extra setup dependencies that could break over time.

Would you be open to a PR that replaces cargo-generate with wasm-pack's --template instead?