[9.x] Universal HigherOrderWhenProxy

[inxilpro]

This PR picks up the work in #37504 and #37561 to unify the behavior of when() and unless() across every implementation by:

• Expanding HigherOrderWhenProxy to support any proxied object, not just Enumerables
• Adding support for HigherOrderWhenProxy in the Conditionable trait
• Replacing the implementation of when() and unless() in EnumeratesValues with Conditionable

It's necessary to implement these changes in 9.x because:

1. There were slight signature differences between the when() method on Enumerable and the when() method on Conditionable (related to type hints)
2. The signature of HigherOrderWhenProxy::__construct needs to be changed slightly to allow for any type of proxied object
3. The return value of EnumeratesValues::when was not defaulted to $this like it is in Conditionable

I want to call out that third item because it is a subtle change.

In Laravel 8.x, the following code will return null. After this PR, the same code will return the Collection with "c" appended to it. I think that is preferable, and means that when() behaves the same way everywhere in the framework.

$results = collect(['a', 'b'])
  ->when(true, function($collection) {
      $collection->push('c');
  });

$results === null; // In Laravel 8
$results instanceof Collection; // In Laravel 9 after this PR

[JosephSilber]

This looks great 👍

Was actually my first thought when I saw the new Conditionable trait.

---

My only gripe is that it feels weird having this now-totally-generic HigherOrderWhenProxy living under the Collections package, but I don't have a better solution ATM.

[driesvints]

@inxilpro this PR seems to have coupled the Support component to the Collections component again because Conditionable is now used in the Collections component. We splitted the Collections out of the Support package for the exact reason to never do this. We'll either need to revert this PR or think of a way to do the conditionable features from this PR differently.

[inxilpro]

Hm. Give me a bit to see if I can come up with something.

[JosephSilber]

Can't Conditionable move under Collections, just like the HigherOrderWhenProxy?

[inxilpro]

Can't Conditionable move under Collections, just like the HigherOrderWhenProxy?

Yeah, that would work. See my comment on #38389 — the only issue is that it doesn't exactly "belong" in the collections package, since it's more general than that, but moving it into that package is the simplest solution.

[franzliedke]

Would it be possible to execute the tests for each package in isolation?

That could help noticing such hidden dependencies more quickly. 🤔