fix: Use ESM build only for browser bundlers (not Node.js)

[amannn]

Note: We can't go full ESM yet, because this requires that upstream dependencies are ESM too (Next.js isn't). These imports would otherwise fail (e.g. because they don't use explicit file extensions).

If we attempt adding ESM support again in the future, consider:

1. We have verbose error messages in dev mode, therefore make sure they can be stripped from a production build.
2. Node doesn't read module, only exports
3. TS has different module resolution algorithms that need to be tested (moduleResolution: "node16")
4. Make sure bundle size doesn't grow (neither of the library, nor when compiled in examples)

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