Export mediumZoom correctly as ESM style

[hmsk]

Summary

This PR fixes this compile error on TypeScript 3.x.

The actual exporting of mediumZoom is taking ESM way since last August.
https://github.com/francoischalifour/medium-zoom/blob/master/src/index.js#L4

TS expects the same way for typedefs.

export = Something on TS

https://www.typescriptlang.org/docs/handbook/modules.html

When exporting a module using export =, TypeScript-specific import module = require("module") must be used to import the module.

[francoischalifour]

Awesome, thank you for the fix!