[8.x] Add query builder map method

[reinink]

This PR adds a new map() method to the Query/Builder and Eloquent/Builder classes (via the BuildsQueries trait). This method is similar to the each() query builder method, where it automatically chunks over the results. Here's how to use it:

return User::orderBy('name')->map(fn ($user) => [
    'id' => $user->id,
    'name' => $user->name,
]), 25);

The primary motivation here is to make it easier to chunk through query results while also performing a map. Previously, this was quite verbose to do (and required a temporary variable):

use Illuminate\Support\Collection;

$users = Collection::make();

User::orderBy('name')->each(function ($user) use ($users) {
    $users->push([
        'id' => $user->id,
        'name' => $user->name,
    ]);
}, 25);

return $users;

For what it's worth, I probably would have preferred the $count argument first (before the $callback), like the chunk() method, but I did it this way to match the each() method.

[taylorotwell]

Merged.