phpstan: Use standard config path

This allows developer to create their own own config file, e.g. for setting `editorUrl`:
https://phpstan.org/user-guide/output-format#opening-file-in-an-editor

phpstan.dist.neon

@@ -0,0 +1,15 @@
+parameters:
+    level: 1
+    paths:
+        - src
+        - tests
+
+    # https://github.com/phpstan/phpstan/issues/694#issuecomment-350724288
+    bootstrapFiles:
+        - vendor/bin/.phpunit/phpunit/vendor/autoload.php
+
+    checkMissingIterableValueType: false
+
+includes:
+    - vendor/phpstan/phpstan-phpunit/extension.neon
+    - vendor/phpstan/phpstan-phpunit/rules.neon