Use latest deepmerge convenience functions

[matatk]

The legacy array merge function should be updated to use the latest API provided by TehShrike/deepmerge#85.

Current Landmarks build script code:

landmarks/scripts/build.js

Lines 375 to 390 in 859c21b

	function legacyArrayMerge(target, source, options) {
	const destination = target.slice()
	source.forEach(function(e, i) {
	if (typeof destination[i] === 'undefined') {
	const cloneRequested = options.clone !== false
	const shouldClone = cloneRequested && options.isMergeableObject(e)
	destination[i] = shouldClone ? clone(e, options) : e
	} else if (options.isMergeableObject(e)) {
	destination[i] = merge(target[i], e, options)
	} else if (target.indexOf(e) === -1) {
	destination.push(e)
	}
	})
	return destination
	}

This needs to match the reference code from deepmerge: arrayMerge example: combine arrays.