Function arrayInsert

contributes/src/vs/base/common/arrays.ts:504–508

(target: T[], insertIndex: number, insertArr: T[])

 * Please don't touch unless you understand https://jsperf.com/inserting-an-array-within-an-array
 */
export function arrayInsert<T>(target: T[], insertIndex: number, insertArr: T[]): T[] {
	const before = target.slice(0, insertIndex);
	const after = target.slice(insertIndex);
	return before.concat(insertArr, after);
}

/**
 * Uses Fisher-Yates shuffle to shuffle the given array