Fix compatibility with Node16/NodeNext moduleResolution

[AsyncBanana]

When setting ModuleResolution to NodeNext in a project's tsconfig.json and using ESM within that project, superstruct's types fail to be discovered. I am fairly sure this is due to the fact that index.d.ts imports other type declarations without file extensions.

It would be a fairly simple fix to change imports to use .js file extensions, which do work, even for type declarations, and shouldn't break anything.

[mnahkies]

Duplicates #1160

Would be nice to get a resolution to this / #1160 - just ran into this today 🤞

[ciscoheat]

Same here, cannot integrate Superstruct in Superforms v2 due to this.

[ciscoheat]

Opened a PR for this issue in #1211

[nerochiaro]

I am testing version 1.0.5-0 which, as I understand it, is a pre-release for the fix to this issue.
It works OK if I put "moduleResolution":"node16" in my tsconfig.json.

However it seems to fail for me when I also use "module": "node16" along with it.
This is what is recommended for node 20 by https://www.npmjs.com/package/@tsconfig/node20 so I would expect superstruct to work with this common configuration. But maybe I am misunderstanding something.

My index.ts file is like this:

import { object, string } from 'superstruct';
const User = object({ foo: string() });

and I get this error:

index.ts:1:32 - error TS1479: The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. Consider writing a dynamic 'import("superstruct")' call instead.
  To convert this file to an ECMAScript module, change its file extension to '.mts', or add the field `"type": "module"` to '/home/ugo/superstructure-test/package.json'.

1 import { object, string } from 'superstruct';

Am I missing something or should this work too ?