Provide a TypeScript definition file for the JS engine

We provide a [VTL engine for several versions of the grammar](https://github.com/InseeFr/VTL-Tools/tree/master/packages) in the shape of NPM packages.

Nowadays, JS packages often come along with a [TypeScript definition](https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html), providing type description and compiler features for the developper using TypeScript (see for example the [type ](https://www.npmjs.com/package/@types/react) [description ](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts)for React).

So it would be nice to provide such definitions for the various versions of the engine (and publish it via the [DefinitivelyTyped repository](https://github.com/DefinitelyTyped/DefinitelyTyped)).