Creating a Types Declaration file for an existing library

Question

I'm working towards building a Type Declaration file for an existing JavaScript library. Since I have no experience with creating declaration files I've created a simple test project that emulates the library I ultimately want to create a declaration file for. I have some TypeScript experience and I've read through the TypeScript documentation on declaration files but I'm struggling to get my test project working. When I try to compile I get the error:

File '<...>/test.d.ts' is not a module.

My test project structure is as follows:

Project
    ├ index.ts
    ├ test.d.ts
    ┕ test.js

test.js

class MyClassA {
    constructor(){}
    
    static staticMethodA(x, y) {
        console.log(`staticMethodA(${x}, ${y})`);
    }
    
    regularMethodA(z) {
        return `regularMethodA(${z})`;
    }
}

class MyClassB {
    constructor(){}
    
    regularMethodB(q) {
        return `regularMethodB(${q})`;
    }
}

var classes = {
    ClassA: MyClassA,
    MyClassB: MyClassB
};

exports.classes = classes;

test.d.ts

declare module "test" {
    class MyClassA {
        constructor();
        public static staticMethodA(x: number, y: number): void;
        public regularMethodA(z: string): string;
    }

    class MyClassB {
        constructor();
        public regularMethodB(q: string): string;
    }

    export interface classes {
        ClassA: MyClassA;
        MyClassB: MyClassB;
    }
}

index.ts

import {MyClassA} from "./test.js";

const foo: MyClassA = new MyClassA();

console.log(foo.regularMethodA("Test"));

I don't know why I'm getting the not a module error. I'm assuming there's a problem with my test.d.ts file but I don't see what I'm missing.