fix: fix TypeScript type definitions for commonjs

This is done by duplicating `index.d.ts` into `index.d.cts`, and
modifying it for CommonJS. The same was done for type tests.

Unfortunately I was unable to find a way to re-use types without
drastically changing the code base.

To test this, a minimum TypeScript version of 4.7 is needed, so it has
been updated. The old types still work with older TypeScript versions.

README.md

@@ -1278,6 +1278,12 @@ try {
 }
 ```
 
+Because axios dual publishes with an ESM default export and a CJS `module.exports`, there are some caveats.
+The recommended setting is to use `"moduleResolution": "node16"` (this is implied by `"module": "node16"`). Note that this requires TypeScript 4.7 or greater.
+If use ESM, your settings should be fine.
+If you compile TypeScript to CJS and you can’t use `"moduleResolution": "node 16"`, you have to enable `esModuleInterop`.
+If you use TypeScript to type check CJS JavaScript code, your only option is to use `"moduleResolution": "node16"`.
+
 ## Online one-click setup
 
 You can use Gitpod, an online IDE(which is free for Open Source) for contributing or running the examples online.