[Proposal] Adding median and mode to collections

[amonger]

I found in a recent project that I needed to calculate the mode in a collection. While you could use an external class, I think it fits well into the collections class as you're already providing functionality which provides an average.

laravel/framework#14305

[Garbee]

If you're going to make a PR, then making an issue here doesn't help much. Just let the PR speak for itself. Those get attention more quickly than Internals issues anyways.