How to import a type from a package and add it to the window object available for the whole project?

Question

This is what I need to do:

import type { Foo } from 'package-foo';

declare global {
  interface Window {
    foo: Foo;
  }
}

I would like it to be accessible from my entire src folder.

How can I do this? Should I use a d.ts file? Triple slash references?