[RFC] Migrate webpack-encore to ESM

[stof]

More and more npm packages are shipping as ESM. In node, the compatibility between ESM and CommonJS has some limitation:

• an ES module can import a CommonJS module
• an ES module can dynamically import a CommonJS module
• a CommonJS module cannot import an ES module (Node 22 has an experimental feature allowing it for synchronous object graphs, i.e. ES modules that don't use top-level await)
• a CommonJS module can dynamically import an ES module through await import(...) (but this can only be used in an async context)

Shipping Encore as a CommonJS module means that we cannot upgrade our dependencies to versions that have migrated to ESM (as Encore.getConfig() is a synchronous API, any solution requiring an async context is a no-go).

Migrating to ESM needs to be done in a new major version as it is a BC break for downstream projects. This will require writing the webpack.config.js file in ESM, either by using type=module in the package.json or by using webpack.config.mjs instead (which is supported by webpack)

[Kocal]

I'm 100% of favor of migrating Encore to ESM, it's something I've also thought recently.

Shipping Encore as a CommonJS module means that we cannot upgrade our dependencies to versions that have migrated to ESM (as Encore.getConfig() is a synchronous API, any solution requiring an async context is a no-go).

Yeah, especially with #985 which forced us to introduce rpc-sync dependency to make ESLint API sync, and here in #1303 where I couldn't upgrade to an ESM version (and #1313 aswell).

Also, do we want to write JavaScript ESM compatible code, or do we want to add an additional build step like... TypeScript?