﻿Given a sorted list of non-overlapping intervals and a new interval, insert the new interval and merge all overlapping intervals. Return the resulting sorted list.

Input: intervals = [[1,3], [6,9], [12,15], [18,20]], new_interval = [5,13]
Expected output: [[1,3], [5,15], [18,20]]

Implement in O(n) time. Include tests covering: insertion at the beginning, end, middle; new interval that merges all existing intervals; new interval that overlaps none; empty initial list.
