Support PHP 7.4 in Drupal 8

I couldn't find any issue about supporting PHP 7.4 in Drupal 8, so here's one.

From some research, there are several dependencies whose current version in HEAD are currently not compatible with PHP 7.4:

• egulias/email-validator - access array on null. Solvable updating to latest version.
• mikey179/vfsstream - use of deprecated curly brackets for array access. Solvable updating to latest version.
• typo3/phar-stream-wrapper - use of deprecated curly brackets for array access. There is currently no stable version with a fix, dev-master is fixed but requires stricter type hinting in Core/Security/PharExtensionInterceptor::assert
• doctrine/annotations - access array on null. The SimpleAnnotiationParser is no longer supported by Doctrine, and is being replaced in #2631202: Doctrine no longer supports SimpleAnnotationReader, incorporate a solution into core. Comment added to notify about PHP 7.4 compatibility at #2631202-114: Doctrine no longer supports SimpleAnnotationReader, incorporate a solution into core
• phpunit/phpunit - Function ReflectionType::__toString() is deprecated. This probably requires updating to PHPUnit 7. #2950132: Support PHPUnit 7 optionally in Drupal 8, while keeping support for ^6.5

Then, beside dependencies, there is Drupal code that needs to be made compatible. #3081386: [META] Fully support PHP 7.4 in Drupal 7 has similar findings. See some:

• RecursiveContextualValidator - Trying to access array offset on value of type int
• Drupal\Core\Controller\ArgumentResolver\RawParameterValueResolver - Deprecated function: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead
• LanguageServiceProvider - Trying to access array offset on value of type bool
• Drupal\Core\Render\Element - Trying to access array offset on value of type int
• Array and string offset access syntax with curly braces is deprecated in /var/www/html/core/modules/views/src/Plugin/views/filter/StringFilter.php on line 344
• Trying to access array offset on value of type null - /var/www/html/core/modules/views/src/Plugin/views/display/PathPluginBase.php:104
• Array and string offset access syntax with curly braces is deprecated in /var/www/html/core/modules/rest/src/RequestHandler.php on line 333