Remove laravel/helpers dependency

The laravel/helpers package is described to "provide a backwards compatibility layer for Laravel 5.8 helpers in the latest Laravel release."

Along the years some of the functions defined in that package were added as native PHP functions, with different implementations.

Some examples are:

• str_contains()
  • Added in PHP 8.0
  • Its $needle argument only supports ?string, while laravel/helpers's supports an array
• array_first()
  • To be added in PHP 8.5
  • Laravel recently moved to use it in several places, through the symfony/polyfill-php85 package
  • PHP's native implementation only takes one argument, while laravel/helpers's takes 3
• array_last()
• Same remarks as array_first()

The only helper from the laravel/helpers package still use by this package was str_start.

But for packages requiring this package, they implicitly require the laravel/helpers package, and if it gets loaded before symfony/polyfill-php85 their implementation gets precedence.

And then, an infinite loop is triggered when calling one of the aforementioned helpers.

For example, laravel/helpers's array_last helper, just calls Arr::last() (from ìlluminate/support), and that was recently changed to use the native array_last function.

Starting on PHP 8.5, that would be fine, but by now, it creates an infinite loop as the laravel/helpers's version is loaded first.

This MR

• Replaces the laravel/helpers dependency by ìlluminate/support which was already implicitly required by the former
• Changes the Message::mentions() method to use the Str::start() method from the Illuminate\Support\Str class, instead of the str_start() helper from the laravel/helpers package