Update/modernize composer.json file

[slaFFik]

Add config.platform.php, update require.php, add some additional params.

[imath]

In 12852:

Update Composer dependencies to match its v2 requirements

• Updates composer/installers to 1.10
• Updates phpcompatibility/phpcompatibility-wp to 2.1
• Updates dealerdirect/phpcodesniffer-composer-installer to 0.7.1
• Updates wp-phpunit/wp-phpunit to latest
• Adds a dependency to phpunit/phpunit v7.5+ to be able to run PHP 8.0 unit tests using @wordpress/env.

Props slaFFik

Fixes #8381
Fixes #8420