Build: add PHPStan: https://github.com/phpstan/phpstan

PHPStan is a static analyser tool for PHP. Let's see how well it works for WordHat.
You can use `composer commit` as a pre-commit hook if you're contributing to the core project.

.editorconfig

@@ -10,7 +10,7 @@ insert_final_newline = true
 trim_trailing_whitespace = true
 indent_style = space
 
-[{.jshintrc,*.json,*.yml,*.xml,*.sh,*.conf}]
+[{.jshintrc,*.json,*.yml,*.xml,*.sh,*.conf,*.neon}]
 indent_style = space
 indent_size = 2
 

composer.json

@@ -30,17 +30,20 @@
     "wp-cli/wp-cli" : "~1.3",
     "joomla-projects/selenium-server-standalone": "~3.5",
     "behat/mink-goutte-driver": "^1.2",
-    "behat/mink-selenium2-driver": "^1.3"
+    "behat/mink-selenium2-driver": "^1.3",
+    "phpstan/phpstan": "~0.9"
   },
   "suggest": {
     "behat/mink-goutte-driver": "Headless Mink driver",
     "behat/mink-selenium2-driver": "JS-enabled Mink driver (requires Selenium2)"
   },
   "scripts": {
-    "phpcs":  "phpcs --standard=phpcs-ruleset.xml -p -s -v -n src --extensions=php",
-    "phpcbf": "phpcbf --standard=phpcs-ruleset.xml -n src",
+    "phpcs":   "phpcs --standard=phpcs-ruleset.xml -p -s -v -n src --extensions=php",
+    "phpcbf":  "phpcbf --standard=phpcs-ruleset.xml -n src",
+    "phpstan": "vendor/bin/phpstan analyse src --level=8",
     "commit": [
-      "@phpcs"
+      "@phpcs",
+      "@phpstan"
     ]
   }
 }