Add "static analysis" Composer keyword (#278)

As per https://getcomposer.org/doc/04-schema.md#keywords by including "static analysis" as a keyword in the `composer.json` file, Composer 2.4.0-RC1 and later will prompt users if the package is installed with `composer require` instead of `composer require --dev`. See composer/composer#10960 for more info.

The "phpcs" keyword is to match other coding-standards packages.

Author: GaryJones

composer.json

@@ -2,6 +2,7 @@
     "name": "sirbrillig/phpcs-variable-analysis",
     "description": "A PHPCS sniff to detect problems with variables.",
     "type": "phpcodesniffer-standard",
+    "keywords" : [ "phpcs", "static analysis" ],
     "license": "BSD-2-Clause",
     "authors": [
         {