usage

import proj4 from "proj4";
import proj4FullyLoaded from "proj4-fully-loaded";
import proj4AnotherOne from "proj4-another-one";

// target is the proj4 that will be updated
const target = proj4;

// sources supply extra definitions
const sources = [proj4FullyLoaded, proj4AnotherOne];

// adds the projection definitions from the sources to the target
merge(target, sources);