read `package.json` `type` to determine output `.mjs` or `.js`

[kecrily]

To that end, I think we could update @eslint/create-config to read an existing package.json to determine the type and output eslint.config.js when the project is ESM and eslint.config.mjs only when it's not.

Originally posted by @nzakas in #20129

[JoshuaKGoldberg]

👍 this would be great. There are several existing npm packages that can do this.

[aladdin-add]

it's the same as how it's working now:

create-config/lib/config-generator.js

Lines 148 to 150 in 76fc885

	const isESMModule = isPackageTypeModule(this.packageJsonPath);
	let configExt = isESMModule ? "js" : "mjs";

am i missing something?

[shoulders]

as an end user, and amateur, when i run eslint --init a eslint.config.mjs is always created.

There is no type setting in my package.json

[kecrily]

I didn't double-check this. Apparently we already have this functionality.