wasm32-unknown-emscripten support

[walkingeyerobot]

💡 Feature description

Add support for wasm32-unknown-emscripten. Support is being added to Emscripten incorporate wasm-bindgen output. However, due to how Emscripten is set up, it needs to invoke wasm-bindgen itself. Because cargo does not support binary tool dependencies, that makes it impossible to have cargo build alone run both the compiler and the Emscripten linker. So instead, we can leverage wasm-pack to make sure wasm-bindgen is available before link time.