Added push() method

[Plytas]

I was looking for a fluent way to add items to collection but wasn't able to find one.
Current alternatives:

SimpleData::collection()->toCollection()->push('foo'); //Technically works if underlying collection is `\Illuminate\Support\Collection`, but cannot be properly type hinted because `$items` is `\Illuminate\Support\Enumerable`.

$collection = SimpleData::collection();
$collection[] = 'foo'; //This works because `DataCollection` implements `ArrayAccess`

This proposed method is the same as \Illuminate\Support\Collection::push().

[rubenvanassche]

In v4 these methods are deprecated since we want to move away from DataCollections to regular Collections, we're not going to add any extra methods in such case just use a Collection from now on.

[Plytas]

Wasn't aware of upcoming v4 changes and I see it has been tagged recently. Thank you!

[rubenvanassche]

Yeah has been in the making for a year, glad I've could finally release it 😄

I hope to completely remove DataCollection in v5 but that requires some extra thinking about how to handle a few scenarios, that's something for the next weeks.