[12.x] Add App::shouldSyncLocale() to keep helpers in line with the app locale

[CasEbb]

When using the Number helper it is possible to specify the locale. The locale's default however, is always English and doesn't change when the app's locale changes. Carbon however, does do this. This leads to the awkward situation where Carbon returns localized results (where applicable), where Number doesn't.

<?php

app()->getLocale(); // "en"
now()->diffForHumans(); // "0 seconds ago"
Number::forHumans(1000); // "1 thousand"

app()->setLocale('nl');
now()->diffForHumans(); // "0 seconden geleden" 🇳🇱 👍
Number::forHumans(1000); // "1 thousand" 🇺🇸 🤔

Carbon's service provider listens for the LocaleUpdated event and updates its locale accordingly. This PR proposes a convience method to the Application class to do the same, as enabling this behaviour by default could be considered as a breaking change. Enabling this behaviour by default may be interesting for the next major version.

[taylorotwell]

Hey @CasEbb - it's a breaking change for us to change the contract on a patch release.