Fix sourcemaps in dev builds

[donmccurdy]

Sourcemaps have been broken in development builds since we began using custom Rollup plugins, as far as I can tell. This PR updates our existing Rollup plugins to preserve source maps, so that we can set debug points in source files (rather than three.module.js) during development.

The update requires npm package magic-string as a dev dependency, 6.4kb minzipped, by the author of Rollup.

Related:

• Rollup: Replace addons three.module.js dependency. #20389 (comment)
• Include sourcemap in the repo? #9941

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
634 kB (157.2 kB)	634 kB (157.2 kB)	+0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
425.2 kB (103.1 kB)	425.2 kB (103.1 kB)	+0 B

[LeviPesin]

For what source maps are used? I never used them personally.

[donmccurdy]

If you're debugging bundled code in your browser's developer tools and can see source files, then your bundler has provided sourcemaps. Sourcemaps allow the browser to display and set breakpoints in the source code, even though compiled code is what's running.

For me this is helpful – knowing I want to debug something in KeyframeTrack.js, I can open the Source tab and jump straight to that file (Cmd + P) rather than searching for line 39,380 in the compiled code.

There's also a link at the bottom of the source file if you want to view the compiled code instead.

[donmccurdy]

Going to merge — This PR fixes the errors shown in the logs during a build, and makes it much easier for me to debug code in the renderer (required for #25909).