[12.x] Determine if the collection contains a perfect number of items

[daniser]

Counting items is one of the hardest challenges of modern programming.
This PR will make a developer's life a little bit easier by telling him if collection contains exactly 42 items.

Why 42?

We all know the answer.

What if I need to check if collection contains N number of items?

This is out of scope of this PR, but don't hesitate! I'm sure Taylor will be happy to accept another PR of this kind😉

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!