Add PHP 8.4 compatibility changes

[JPry]

When running slic with PHP 8.4, I noticed the following notices:

❯ slic use the-events-calendar 

Deprecated: StellarWP\Slic\args(): Implicitly marking parameter $source as nullable is deprecated, the explicit nullable type must be used instead in /Users/jpry/projects/slic/src/utils.php on line 22

Call Stack:
    0.0001     398368   1. {main}() /Users/jpry/.composer/bin/slic:0
    0.0006     398784   2. {closure:/Users/jpry/.composer/bin/slic:4-14}() /Users/jpry/.composer/bin/slic:14
    0.0011     424440   3. require_once('/Users/jpry/projects/slic/slic.php') /Users/jpry/.composer/bin/slic:13


Deprecated: StellarWP\Slic\check_status_or(): Implicitly marking parameter $else as nullable is deprecated, the explicit nullable type must be used instead in /Users/jpry/projects/slic/src/process.php on line 240

Call Stack:
    0.0001     398368   1. {main}() /Users/jpry/.composer/bin/slic:0
    0.0006     398784   2. {closure:/Users/jpry/.composer/bin/slic:4-14}() /Users/jpry/.composer/bin/slic:14
    0.0011     424440   3. require_once('/Users/jpry/projects/slic/slic.php') /Users/jpry/.composer/bin/slic:13
    0.0047     526440   4. require_once('/Users/jpry/projects/slic/src/utils.php') /Users/jpry/projects/slic/slic.php:6


Deprecated: StellarWP\Slic\ensure_wordpress_ready(): Implicitly marking parameter $version as nullable is deprecated, the explicit nullable type must be used instead in /Users/jpry/projects/slic/src/wordpress.php on line 154

Call Stack:
    0.0001     398368   1. {main}() /Users/jpry/.composer/bin/slic:0
    0.0006     398784   2. {closure:/Users/jpry/.composer/bin/slic:4-14}() /Users/jpry/.composer/bin/slic:14
    0.0011     424440   3. require_once('/Users/jpry/projects/slic/slic.php') /Users/jpry/.composer/bin/slic:13

slic version 1.9.1 - StellarWP local testing and development tool

Using the-events-calendar

This PR resolves these particular deprecated notices, and handles a few other small tweaks:

1. Replaces a ternary operator with ??.
2. Import Closure instead of using the leading backslash
3. Remove an unused parameter (related to the last deprecation notice)