OP-352 - Add phpstan configuration file

phpstan.neon

@@ -0,0 +1,17 @@
+parameters:
+    level: 8
+    reportUnmatchedIgnoredErrors: false
+    paths:
+        - src
+
+    excludePaths:
+        # Sitemap dependent providers
+        - 'src/SitemapProvider'
+        - 'src/Importer/AbstractImporter'
+        - 'tests/Application/config/bootstrap.php'
+        - 'tests/Fixture/PageFixtureTest.php'
+        - 'src/Controller/Helper'
+    ignoreErrors:
+        - identifier: missingType.iterableValue
+        - identifier: missingType.generics
+        - '#.*NodeParentInterface.*#'