Provide a TypeScript definition file for the JS engine

[romaintailhurat]

We provide a VTL engine for several versions of the grammar in the shape of NPM packages.

Nowadays, JS packages often come along with a TypeScript definition, providing type description and compiler features for the developper using TypeScript (see for example the type description 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).

[NicoLaval]

antlr4 seems to have a TS package: antlr4ts