Lock nikic/php-parser v4

It seems that both phpunit and php-code-coverage have
started to accept nikic/php-parser v5, but that's leading
to problems with PHP 7.4 runs, that we still support.

So, with this commit, we are locking nikic/php-parser v4

Once we officially make moodle-cs to only support php >= 8.0
we can remove this lock. Issue: moodlehq#88

composer.json

@@ -46,6 +46,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^9.6",
+        "nikic/php-parser": "^4.18",
         "mikey179/vfsstream": "^1.6",
         "overtrue/phplint": "^3.4.0 | ^9.0.4",
         "phpmd/phpmd": "^2.11",