[5.4] Add "size" method for initializing collections

[calebporzio]

Similar to #18608, this method can be used to initialize a collection of a specific length for further manipulation down the pipeline.

$arrayOfFiveSomethings = Collection::size(5)->map(function () {
  return $something;
})

[CristianLlanos]

I believe you can achieve this behaviour with the times method 🤔 recently merged #18457

[taylorotwell]

See above comment.