Builds the dicEx frontend assets (Three.js + Rapier) into
priv/static/dic_ex.min.js so they ship with the Hex package.
mix dic_ex.buildRun inside the package when you change anything under assets/src. Consumers
of the published package only need the prebuilt file plus the LiveView hook.
Prerequisites
Requires Node.js and runs pnpm install (or npm install) on first use when
assets/node_modules is missing.