Specify type field in package.json

[mattt]

Related to #191

From publint.dev

Since Node.js v20.10.0, it introduces a new --experimental-default-type flag to flip the default module system from "CJS-as-default" to "ESM-as-default". If enabled, package.json without the "type" field will mean its descendant JS files to be interpreted as ESM instead of CJS, which may not work correctly.

While this only applies to files outside of node_modules, it's still recommended to set it up for future-proofing. And it also helps the --experimental-detect-module flag if enabled.

Hence, if you've not set the "type" field, you can explictly set it as "type": "commonjs" (default value), or migrate to "type": "module" and write in ESM completely if possible.