[12.x] Add keyByListValues array helper

[daniser]

This PR adds new keyByListValues array helper function.
This function keys array list entries by its values while preserving its associative part, thus making array 100% associative.
Newly keyed entries' values are replaced with some default value (2nd argument).
The caller is responsible for list entries to be valid array keys.

(I don't know if terminology is correct because AFAIK arrays can be either associative or non-associative, not both)

Suggested use case is when some optional parameter(s) may be associated with array entry (likely class name or some identifier):

$middleware = [
    EnsureUserHasRole::class => ['role' => 'editor'],
    SomeOtherMiddleware::class,
];

Arr::keyByListValues($middleware, []);

/*[
    EnsureUserHasRole::class => ['role' => 'editor'],
    SomeOtherMiddleware::class => [],
];*/

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!