Until emscripten-core/emscripten#11792 is fixed, there are a few steps to getting an ES6 module working in node and in the browser. This workaround builds two separate sets of files, out-node.{mjs,wasm} and out-web.{mjs,wasm}.
First you'll need to install Emscripten and make the em++ etc. commands available in your shell.
To build for web, use make web and look at example.html.
To build for node, use make node and look at example.mjs along with pre_js.js and post_compile_mjs.
Thanks to https://github.com/lucydsl/liblucy which was helpful in figuring out this workaround.