Specify the cron array return type

[oiahoon]

Summary

• add a PHPStan return-type override for _get_cron_array()
• regenerate the matching WordPress stub annotation
• add a focused type-inference regression test for the nested cron event shape

Fixes #269.

Testing

• php -d memory_limit=-1 vendor/bin/phpunit --filter 'get-cron-array\.php' --testdox
• full PHPUnit suite: 1,442 tests, 1,612 assertions
• full PHPStan analysis in serial debug mode
• PHPCS
• stub generation and PHP syntax checks
• git diff --check

[szepeviktor]

This is a nice PR.
Thank you.