Add PHPStan and PHP Mess Detector for improved code quality

phpstan.neon

@@ -0,0 +1,19 @@
+parameters:
+    level: 5
+    paths:
+        - includes
+        - admin
+        - wp-plugin-starter-template.php
+    excludePaths:
+        - vendor
+        - node_modules
+        - tests
+        - bin
+        - build
+        - dist
+    ignoreErrors:
+        - '#Function apply_filters invoked with [0-9]+ parameters, 2 required.#'
+        - '#Function do_action invoked with [0-9]+ parameters, 1 required.#'
+        - '#Function add_action invoked with [0-9]+ parameters, 2 required.#'
+        - '#Function add_filter invoked with [0-9]+ parameters, 2 required.#'
+    reportUnmatchedIgnoredErrors: false