internal: add phpstan to dev dependencies, create config

phpstan.neon

@@ -0,0 +1,31 @@
+parameters:
+    level: 8
+    paths:
+        - src
+        - tests
+    scanFiles:
+        # Pest handles loading custom helpers only when running tests
+        # @see https://pestphp.com/docs/helpers#usage
+        - tests/Pest.php
+        # Mockery doesn't autoload its helper functions
+        - vendor/mockery/mockery/library/helpers.php
+    ignoreErrors:
+        # Pest implicitly binds $this to the current test case
+        # @see https://pestphp.com/docs/underlying-test-case
+        - message: '#^Undefined variable: \$this$#'
+          paths:
+            - tests/*
+        # Pest custom expectations are dynamic and not conducive static analysis
+        # @see https://pestphp.com/docs/expectations#custom-expectations
+        - messages:
+            - '#Call to an undefined method Pest\\Expectation|Pest\\Support\\Extendable::#'
+            - '#Call to an undefined method Pest\\Mixins\\Expectation#'
+            - '#Call to an undefined method Pest\\PendingCalls\\TestCall|Pest\\Support\\HigherOrderTapProxy::#'
+          paths:
+            - tests/*
+        # Some mockery errors
+        - messages:
+            - '#Call to an undefined method Mockery\\ExpectationInterface|Mockery\\HigherOrderMessage::#'
+            - '#Parameter .+ Mockery\\LegacyMockInterface given.#'
+          paths:
+            - tests/*