Type definitions for Typescript

[manu2504]

It would be great if this package had a corresponding type declaration file for Typescript users.

There's currently one such file for leaflet (https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/leaflet).
Having one for leaflet-color-number-markers as well would allow users to get past the Typescript compilation error "TS7016: Could not find a declaration file for module 'leaflet-color-number-markers'."