Set up GitHub Actions

psalm.xml

@@ -1,9 +1,9 @@
 <?xml version="1.0"?>
 <psalm
-        totallyTyped="false"
         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"
+        errorLevel="2"
 >
     <projectFiles>
         <directory name="src"/>