Improve PHP 8.x linting

The PHPCompatibilityWP package is tied to the 9.x release of the
PHPCompatibility package.

The current 9.3.5 release of PHPCompatibility has not been updated
since December 2019 and I believe is missing quite a few updated
sniffs for PHP 8.x syntax.

See: PHPCompatibility/PHPCompatibility@9.3.5...develop

This labels the PHPCompatibility `develop` branch as a 9.99.99
release, which satisifies the PHPCompatibilityWP requirements and
improves scanning for PHP 8.x compatibility.

See: PHPCompatibility/PHPCompatibilityWP#40 (comment)

Changes:
* `composer config minimum-stability dev`
* `composer require --dev
  phpcompatibility/php-compatibility:"dev-develop as 9.99.99"`
* `composer update phpcompatibility/phpcompatibility-wp`

composer.json

@@ -17,7 +17,8 @@
   "require-dev": {
     "squizlabs/php_codesniffer": "3.*",
     "phpcompatibility/phpcompatibility-wp": "*",
-    "wp-coding-standards/wpcs": "dev-develop"
+    "wp-coding-standards/wpcs": "dev-develop",
+    "phpcompatibility/php-compatibility": "dev-develop as 9.99.99"
   },
 
   "scripts": {
@@ -32,5 +33,6 @@
       "composer/*": true,
       "dealerdirect/phpcodesniffer-composer-installer": true
     }
-  }
+  },
+  "minimum-stability": "dev"
 }