Add Psalm static code analysis

Author: PhrozenByte

.gitattributes

@@ -1,5 +1,6 @@
 /tests          export-ignore
 /.gitattributes export-ignore
 /.gitignore     export-ignore
+/psalm.xml      export-ignore
 
 *.php diff=php

composer.json

@@ -36,6 +36,7 @@
     "require-dev": {
         "phpunit/phpunit": "^8",
         "mockery/mockery": "^1.3",
-        "symfony/yaml": "^5.2"
+        "symfony/yaml": "^5.2",
+        "vimeo/psalm": "^4.4"
     }
 }

psalm.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="4"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+    <issueHandlers>
+        <InternalClass errorLevel="suppress" />
+        <InternalMethod errorLevel="suppress" />
+    </issueHandlers>
+</psalm>