This commit fixes the `wasm-pack build --mode no-install` command from
unconditionally panicking as well as `--mode force`. These steps were
calling an `unwrap()` on an internal `Option<T>` which was supposed to
be set during `step_install_wasm_bindgen`, but that step wasn't run in
these modes. The mode configuration of steps has been refactored
slightly to ensure that more steps are shared between these modes to
reduce duplication.