Invalid .mjs and .cjs extensions

[rtritto]

.mjs and .cjs extensions for ESM and CJS aren't supported.

Dual package

My related issue: mongodb-js/ejson-shell-parser#157

Steps to reproduce

• yarn init -y
• yarn add @preconstruct/cli
• add in package.json

  "main": "dist/<PROJECT_NAME>.cjs",
  "module": "dist/<PROJECT_NAME>.mjs",
  "exports": {
    "import": "./dist/<PROJECT_NAME>.mjs",
    "require": "./dist/<PROJECT_NAME>.cjs"
  }

• create an empty src/index.js file
• yarn preconstruct build

🎁 error <PROJECT_NAME> main field is invalid, found `"dist/<PROJECT_NAME>.cjs"`, expected `"dist/<PROJECT_NAME>.cjs.js"`
🎁 error <PROJECT_NAME> module field is invalid, found `"dist/<PROJECT_NAME>.mjs"`, expected `"dist/<PROJECT_NAME>.esm.js"`
🎁 info Some of the errors above can be fixed automatically by running preconstruct fix
🎁 info If you want to learn more about the above error, check https://preconstruct.tools/errors
🎁 info If the error is not there and you want to learn more about it, open an issue at https://github.com/preconstruct/preconstruct/issues/new
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

<PROJECT_NAME> is field name of package.json