Allow extension methods

[robstoll]

Describe the bug
I would like to create an extension method

To Reproduce

declare module 'js-big-decimal' {
}

Fails with "is a non-module entity"

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
Add any other context about the problem here.

[royNiladri]

Hi,
You can extend the class as follows:

import bigDecimal from "js-big-decimal";

class bigger extends bigDecimal {
  sayHi(text: string) {
    console.log("Say hi ", text);
  }
}

const b = new bigger("232.54");
console.log(b.getPrettyValue());
console.log(b.sayHi("Nil"));

Which can be run normally: