JavaScript: defining custom global variable

Hello colleagues, 

In order to get rid of "Unknown Variable" warnings and to make development experience better, I want to set up the type definitions for custom global variables. I have the following scenario. I'm using Jest together with jest-puppeteer. I have successfully installed the type definitions for jest itself (through preferences -> Languages and Frameworks -> JavaScript -> Libraries) and warnings related to global Jest functions disappeared. However jest-puppeteer defines more globally accessible variables. One such variable is `page` that is central for testing with Puppeteer.  

The types for this global variable can be found together with other Puppeteer type declarations: https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/puppeteer

However I could not find, how to configure one instance of `page` to be a global variable. I assume I need to define my own type definition file with that global variable and use it for WebStorm config, but I need some help on how to do that exactly.