Add PHP 8 support

[macburgee1]

This PR seeks to add PHP 8 support to composer.json and to update tests for compatibility.

A code scan doesn't reveal any compatibility issues:

$ vendor/squizlabs/php_codesniffer/bin/phpcs --standard=PHPCompatibility src/ --runtime-set testVersion 8.0
$

$ vendor/squizlabs/php_codesniffer/bin/phpcs --standard=PHPCompatibility tests/ --runtime-set testVersion 8.0
$

$ vendor/squizlabs/php_codesniffer/bin/phpcs --standard=PHPCompatibility scripts/ --runtime-set testVersion 8.0
$

Tests pass successfully post-update on both PHP 7.3 and PHP 8.0.

Tests with PHP 7.3:

$ composer test
> phpunit --colors --configuration tests/phpunit.xml
PHPUnit 9.5.0 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.3.16
Configuration: tests/phpunit.xml
Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

...............                                                   15 / 15 (100%)

Time: 00:00.251, Memory: 6.00 MB

OK (15 tests, 25 assertions)

Tests with PHP 8.0:

$ composer test
> phpunit --colors --configuration tests/phpunit.xml
PHPUnit 9.5.0 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.0.0
Configuration: tests/phpunit.xml
Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

...............                                                   15 / 15 (100%)

Time: 00:01.768, Memory: 4.00 MB

OK (15 tests, 25 assertions)

Note: PHP compatibility will need to be addressed in dependencies before php-css-lint can be installed on PHP 8; however. To run tests in PHP 8, I installed with PHP 7.3 and then swapped out versions in DDEV.

[neilime]

Hi, thank you for this PR @macburgee1