chore: merge the app with the container & implement the ApplicationContract (#3862)

* chore: merge the app with the container & implement the ApplicationContract

Illuminate components always expect the app to be the container, but also expect the app to be implementing the laravel app contract. This means that very often between minor illuminate updates we get a call to a method on the app that doesn't exist in the Flarum app. This fixes the issue once and for all.

* chore: improve concern implementation readability
* chore: service provider no longer has to change app type
* chore: unimplement `terminat(e/ing)`
* Apply fixes from StyleCI
* chore: recover `container` prop
* chore: return types
* fix: phpstan errors

php-packages/testing/src/integration/TestCase.php

@@ -46,6 +46,8 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
     protected function app()
     {
         if (is_null($this->app)) {
+            $this->config('env', 'testing');
+
             $bootstrapper = new Bootstrapper(
                 $this->config,
                 $this->extensions,