feat: respect file extensions defined in package.json

[kricsleo]

resolves #373
resolves #508

Converting to a draft PR for now, because I want to address the common output extensions issue.

[kricsleo]

Currently, the output file extensions (.mjs, .cjs) are fixed. This PR proposes inferring the entryFileNames and chunkFileNames extensions based on the package.json's entries config.

Motivation:

This allows users to customize output extensions to match their specific needs, addressing scenarios where the default .mjs and .cjs are not desired.

Example:

Given:

"exports": {
  ".": {
    "require": "./dist/index.js",
    "import": "./dist/index.mjs"
  }
}

The output would be:

dist
├── index.js   (cjs)
└── index.mjs  (esm)

Instead of the current:

dist
├── index.cjs  (cjs)
└── index.mjs  (esm)

Similarly:

"exports": {
  ".": {
    "require": "./dist/index.cjs",
    "import": "./dist/index.js"
  }
}

should generate:

dist
├── index.cjs (cjs)
└── index.js (esm)

Benefits:

This can resolve #508 (only concerned with esm).

Additionally, #373 (iife) can also be resolved because users can easily specify:

"main": "./dist/index.js"

so that the iife outputs also meet requirements.

I'd love to get feedbacks on this 🙏🏻.

[pi0]

unbuild is strictly opinionated about having .cjs/.mjs in output as it helps toolings to explicitly know the type of files without need to lookup for package.json (which at least has filesystem IO cost).

iife support is also non goal since unbuild is made for ESM (+CJS compat)

Thanks for PR ❤️

[kricsleo]

Oh, I also agree with this point; appropriate restrictions lead to a better ecosystem. 👏🏻