Sort of get config to work

config/bundles.php

@@ -0,0 +1,5 @@
+<?php
+
+return [
+    Pcm\IconBundle\PcmIconBundle::class => ['all' => true],
+];

config/packages/pcm_icon.yaml

@@ -0,0 +1,19 @@
+when@dev:
+    pcm_icon:
+        directories:
+            - '%kernel.project_dir%/public/icons'
+        palletes: []
+
+when@prod:
+    pcm_icon:
+        directories:
+            - '%kernel.project_dir%/public/icons'
+        palletes: []
+
+when@test:
+    pcm_icon:
+        directories:
+            - '%kernel.project_dir%/tests/icons'
+        palletes:
+            - []
+

config/services.yaml

@@ -4,13 +4,12 @@ services:
         autoconfigure: true
 
     pcm_icon.icon_extension:
-        alias: Pcm\IconBundle\Twig\Functions\IconExtension
         public: true
-
+        class: Pcm\IconBundle\Twig\Functions\IconExtension
+        arguments:
+            $directories: []
+            $palletes: []
 
     Pcm\IconBundle\Twig\Functions\IconExtension:
-        tags: ['twig.extension']
         public: false
-        arguments:
-            $directories:
-                - '%kernel.project_dir%/public/icons'
+        alias: pcm_icon.icon_extension