refactor: let #[Schedule] run without Laravel

[ogorzalka]

First step of extracting pollora/schedule (roadmap, medium term: "keep only WP-Cron in the package"). Nothing moves yet.

What changes

• ScheduleDiscovery, the part of the module that runs on WP-Cron alone, no longer uses Illuminate\Support\Str and class_basename(): it derives default hook names itself (NightlyReports::compile() → nightly_reports_compile). A parity test pins the result to Str::snake() on several shapes of names, so hooks already scheduled under a derived name keep it.
• tests/Unit/Schedule/ScheduleDiscoveryTest.php: the first tests that run the discovery: each method scheduled on its hook (named or derived) with its recurrence and arguments, the schedules WordPress does not know (Every::MONTH, Interval) added through cron_schedules, the method run with its arguments when the hook fires.
• tests/Unit/Schedule/PackageBoundaryTest.php: ScheduleDiscovery may only use the attributes, discovery, PSR logging and WP-Cron. It fails on the previous code (also uses 'Illuminate\Support\Str').

The bridge to Laravel's scheduler (wordpress.use_laravel_scheduler: Scheduler, its events and jobs, the wp_events table) is Laravel by nature and stays in the framework.

Checks

1888 tests, PHPStan 0, Pint and Rector clean. Independent of #464 and #465.