[12.x] Add prepend option for Str::plural()

[caseydwyer]

When we reach for the Str::plural() helper, we frequently want to prepend the string itself with the $count that's being passed in. That typically looks something like this:

We had {{ number_format($attendees->count()) . ' ' . Str::plural('attendee', $attendees->count()) }} at Laracon 2025.

This PR cleans up the repetition a bit, by adding an optional prepend parameter to the Str::plural() method. Here's what the above looks like, when using that option:

We had {{ Str::plural('attendee', $attendees->count(), prepend: true) }} at Laracon 2025.

The parameter defaults to false, so a non-breaking change. Tests included, and added a couple of extras that I think are mostly covered by the Pluralizer...but thought it might be helpful to ensure we don't get any unexpected $count prepending when this new param is not in use, since the Pluralizer isn't aware of that piece. Let me know if there's anything I can change or help with, here! Happy to update docs as well, if this gets merged.