fix: Collection::flatMap callback can return Enumerable

[bastien-phi]

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md

Fixes #969

Changes

This PR allows PHPStan to resolve TReturn when flatMap is called with a callback returning an Enumerable.

For exemple, before this PR:

$users = User::all();
$accounts = $users->flatMap(fn(User $user) => $user->accounts);

fails with

Unable to resolve the template type TReturn
in call to method Illuminate\Support\Collection<int,App\Model\User>::flatMap()

After this PR, $accounts is successfully resolved as lluminate\Support\Collection<int,App\Model\Account>.

[szepeviktor]

Thank you!

[canvural]

Thank you! I just added one feedback regarding the test case.

[bastien-phi]

Is it better this way ?

[canvural]

Thank you!

[bastien-phi]

You're welcome!