Create a function that takes an array of date objects and return the "longest streak" (i.e. longest number of consecutive days in a row).

Example

longestStreak([
  [
    date => "2019-09-18"
  ],
  [
    date => "2019-09-19"
  ],
  [
    date => "2019-09-20"
  ],
  [
    date => "2019-09-26"
  ],
  [
    date => "2019-09-27"
  ],
  [
    date => "2019-09-30"
  ]
]) ➞ 3

Notes

• An empty array should return 0.
• In PHP, the array will consist of associative arrays, each with a key date.