Adding Registering alias feature to the package

[nyamsprod]

This PR is inspired by my work on bakame/cron to resolve #64

here's the proposed public API:

<?php

use Cron\CronExpression;

if (!CronExpression::supportsAlias('@every')) {
    CronExpression::registerAlias('@every', '* * * * *');
}

CronExpression::getAliases();
// returns
// array (
//   '@yearly' => '0 0 1 1 *',
//   '@annually' => '0 0 1 1 *',
//   '@monthly' => '0 0 1 * *',
//   '@weekly' => '0 0 * * 0',
//   '@daily' => '0 0 * * *',
//   '@midnight' => '0 0 * * *',
//   '@hourly' => '0 * * * *',
//   '@every' => '* * * * *',
// )
CronExpression::supportsAlias('@foobar'); //return false
CronExpression::supportsAlias('@daily');  //return true
CronExpression::supportsAlias('@every');  //return true

$cron = new CronExpression('@every');  // works!
$cron->getExpression(); // return * * * * *

CronExpression::unregisterAlias('@every'); // return true
CronExpression::unregisterAlias('@every'); // return false 
                                           // because on the second time the alias is already removed!

CronExpression::supportsAlias('@every');   //return false

new CronExpression('@every');  //throws InvalidArgumentException unknown or unsupported expression
CronExpression::unregisterAlias('@daily');  //throws LogicException exception

Comments and remarks are welcomed.
PS: while running the test as is on PHP8.1 some tests were failing prior to my addition via this PR.

[dragonmantank]

Overall this looks good, except the tests are failing. Looks like it's expecting CronExpression::aliases() but it was implemented as CronExpression::getAliases(). That should clear that up.

Can you also rebase/update to the latest master? That should clear up the conflict from some new tests that were added.

Thanks!

[nyamsprod]

PR rebased to master and fix method call in test.

[dragonmantank]

Thanks!