[12.x] Add Arr::sole() method

[ralphjsmit]

I was missing an Arr::sole() method when I was working with a simple array for which wrapping in a collection is quite overkill. This PR adds the method, both a simple version without parameters and a version with a callback support same as the other Arr::where() method.

$foo = Arr::sole(['foo']);

$bar = Arr::sole(['bar', 'baz'], fn (string $value) => $value === 'bar');

Thanks!

[SteveEdson]

Ok, this is my new favourite thing in Laravel.