Fix coverage generation by adding php-code-coverage library

Root cause identified: PHPUnit cannot generate coverage reports
without the phpunit/php-code-coverage library. While yoast/wp-test-utils
provides PHPUnit, it doesn't include the code coverage components.

Adding phpunit/php-code-coverage ^9.2 to match PHPUnit 9.6.29.

This will allow PCOV to work properly with PHPUnit to generate
coverage.xml files.

Author: aristath

composer.json

@@ -18,7 +18,8 @@
 		"szepeviktor/phpstan-wordpress": "^2.0",
 		"phpstan/extension-installer": "^1.4",
 		"yoast/yoastcs": "^3.0",
-		"friendsofphp/php-cs-fixer": "^3.75"
+		"friendsofphp/php-cs-fixer": "^3.75",
+		"phpunit/php-code-coverage": "^9.2"
 	},
 	"scripts": {
 		"check-cs": [