Fix Collection::implode with \Stringable objects (with exception for Eloquent models)

[timkelty]

Reimplements #54630, now accounting for Eloquent models, which are \Stringable.

Tests now exist for Colletions with \Stringable, \Illuminate\Support\Stringable, and \Illuminate\Database\Eloquent\Model.

[taylorotwell]

This logic is hard to parse.

[timkelty]

Simplified logic to make it easier to parse.

The bigger question for future versions is whether the test for \Stringable or \Illuminate\Support\Stringable should be retained at all.

It seems limiting that Collections of objects that happen to implement \Stringable or \Illuminate\Support\Stringable would have this behavior forced.

[timkelty]

Closing, as this really would constitute a breaking change.

Any Collections with objects implementing \Stringable, relying on a plucked value, could be inadvertently affected.