[1.x] Add ability to resolve feature instance.

[timacdonald]

This PR gives the ability to resolve the implementation of the feature.

class MyFeature
{
    public $name = 'my-feature';

    // ...
}

Feature::define(MyFeature::class);

$instance = Feature::instance('my-feature');
$instance = Feature::instance(MyFeature::class);

The instance method will return either an instance of the feature class, a lottery, or a closure. The tests give a good rundown of when each of these might occur.

[cosmastech]

It would be amazing if we could add an enum check here as well 😄

[timacdonald]

What would that do, @cosmastech?

[cosmastech]

If I misunderstood this, disregard:

Would love to have the ability to call Feature::define(FeatureFlagEnum::NewFeature, fn() => false), but maybe that's not really what this PR is aiming for.