chore: configure PHPStan static analysis (#87)

* chore: configure PHPStan static analysis

* chore: exclude `/phpstan/` from CS ruleset

---------

Co-authored-by: John Parris <john.parris@wpengine.com>

.phpcs.xml.dist

@@ -7,6 +7,7 @@
 	<!-- Exclude composer vendor directory -->
 	<exclude-pattern>/vendor/</exclude-pattern>
 	<exclude-pattern>/node_modules/</exclude-pattern>
+	<exclude-pattern>/phpstan/</exclude-pattern>
 	<exclude-pattern>/tests/</exclude-pattern>
 
 	<!-- How to scan -->

composer.json

@@ -14,12 +14,17 @@
     "phpunit/phpunit": "^9.5",
     "wpengine/wpengine-coding-standards": "dev-develop",
     "yoast/phpunit-polyfills": "^1.0",
-    "squizlabs/php_codesniffer": "^3.7"
+    "squizlabs/php_codesniffer": "^3.7",
+    "phpstan/phpstan": "^1.10",
+    "phpstan/extension-installer": "^1.3",
+    "szepeviktor/phpstan-wordpress": "^1.3",
+    "axepress/wp-graphql-stubs": "^1.14"
   },
   "scripts": {
     "lint": "parallel-lint -e php --no-colors --exclude vendor .",
     "phpcs": "phpcs",
     "phpcs:fix": "phpcbf",
+    "phpstan": "phpstan analyze --ansi --memory-limit=1G",
     "suite": [
         "@lint",
         "@phpcs",
@@ -29,7 +34,8 @@
   },
   "config": {
     "allow-plugins": {
-      "dealerdirect/phpcodesniffer-composer-installer": true
+      "dealerdirect/phpcodesniffer-composer-installer": true,
+      "phpstan/extension-installer": true
     },
     "platform": {
       "php": "7.4"