Add Psalm config

Fix #3037

.gitattributes

@@ -12,6 +12,7 @@
 /phpmd.xml               export-ignore
 /phpstan.neon            export-ignore
 /phpunit.xml.dist        export-ignore
+/psalm.xml               export-ignore
 /.github                 export-ignore
 /tests                   export-ignore
 

psalm.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="5"
+    resolveFromConfigFile="true"
+    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"
+    findUnusedBaselineEntry="true"
+    findUnusedCode="true"
+>
+    <projectFiles>
+        <directory name="src" />
+        <file name="lazy/Carbon/UnprotectedDatePeriod.php" />
+        <file name="lazy/Carbon/TranslatorStrongType.php" />
+        <file name="lazy/Carbon/MessageFormatter/MessageFormatterMapperStrongType.php" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+</psalm>