Composer: update keywords

Since Composer 2.4.0, Composer will prompt users if they are sure they want to install something as `require` instead of `require-dev` if the package contains certain "dev" related keyword, like `static analysis`.

This adds the keywords to this package to help users add the package in the most appropriate place.

Refs:
* https://getcomposer.org/doc/04-schema.md#keywords
* composer/composer#10960

Co-authored-by: Markus Staab <maggus.staab@googlemail.com>

composer.json

@@ -3,6 +3,10 @@
     "description": "This tool checks the syntax of PHP files about 20x faster than serial check.",
     "homepage": "https://github.com/php-parallel-lint/PHP-Parallel-Lint",
     "license": "BSD-2-Clause",
+    "keywords": [
+        "lint",
+        "static analysis"
+    ],
     "authors": [
         {
             "name": "Jakub Onderka",