"module" export should by an .mjs file

[developit]

This is important because right now, dist/preact.esm.js is actually not treated as a strict Module by Webpack 4. You can try this out:

// name this file foo.mjs
import { options } from 'preact';

Running the above through a vanilla Webpack config will resolve preact to node_modules/preact/dist/preact.esm.js, but will then throw an exception since Webpack does not infer strict ESM validity from the package.json "module" fields - it's only inferred from the file extension.