[9.x] Adds Arr::join()

[ecrmnn]

This PR adds join method from Collections to Arr.

$stack = ['Tailwind', 'Alpine', 'Laravel', 'Livewire'];

Arr::join($stack, ', ', ' and ');

// Tailwind, Alpine, Laravel and Livewire

[DarkGhostHunter]

Should the Collection use then Arr::join() instead?