[12.x] Add Arr::soleValue method

[Amirhf1]

This PR adds a soleValue method to the Arr class. This method works similarly to sole, but instead of returning the entire item, it only returns a specific field or key from the sole item.

Since the sole method was recently added to the Arr class in version 12, and a similar soleValue method exists in Query Builder and Eloquent, adding this method to Arr creates greater consistency between Laravel's various APIs.

Changes:

• Add soleValue method to the Arr class
• Add comprehensive tests for the new functionality