Rollup: Replace addons three.module.js dependency.

[mrdoob]

This allows producing custom builds that include addons.

For example, adding OrbitControls:

1. Add export { OrbitControls } from '../examples/jsm/controls/OrbitControls.js'; in ./src/Three.js.
2. Run npm run build

Fixes #20387

[Mugen87]

Unrelated to this PR but related to the rollup build^^: I've noticed lately the following warning when performing the build:

(!) Broken sourcemap
[ROLLUP] https://rollupjs.org/guide/en/#warning-sourcemap-is-likely-to-be-incorrect
[ROLLUP] Plugins that transform code (such as 'at position 5') should generate accompanying sourcemaps

Unfortunately, I'm not sure when this warning appeared for the first time 🤔 .

[DefinitelyMaybe]

I'd love to get into writing up some documentation around folks creating custom builds and/or tweeking three.js for specific applications

[mrdoob]

@Mugen87 Weird, I'm not getting that 🤔

[Mugen87]

When removing build/three.js and build/three.min.js and updating package.json, I've noticed addons() and wondered if we still need it. AFAICS, the helper was originally implemented in 2020 when we have not used import maps yet, see #20387 (comment).

Now with import maps in place and a pure ESM code base, can we simplify rollup.config.js and remove the addons() helper?