Allow to extend the deltaE methods

[latin-1]

import Color from "colorjs.io";
Color.deltaEMethods.deltaECustom = (color, sample) => { ... };

import {deltaEMethods} from "colorjs.io/fn";
deltaEMethods.deltaECustom = (color, sample) => { ... };

[LeaVerou]

Suggested change

	export { deltaE76, deltaECMC, deltaE2000, deltaEJz, deltaEITP, deltaEOK };
	// We want both a default export so it can be extended
	// as well as individual named exports
	export {
	deltaE76,
	deltaECMC,
	deltaE2000,
	deltaEJz,
	deltaEITP,
	deltaEOK
	};

[LeaVerou]

This seems ok (and is running this time!). Thanks!