Switch to PNPM

I propose we switch to PNPM like we are about to in H5Web with the new monorepo structure. PNPM's node_modules structure is great at avoiding version conflicts, particularly for us with ESLint plugins that are shared between eslint-config-galex and CRA. It's also a lot faster than NPM and somewhat safeguards against phantom dependencies (dependencies that are used in a package but not installed).

I've added a PNPM cheat sheet to the CONTRIBUTING guide, but it's very similar to NPM.