Issue #3398284 by AstonVictor: Create schema config

3398284 - Add config

config/schema/prevnext.schema.yml

+prevnext.settings:
+  type: config_object
+  label: 'PrevNext settings'
+  mapping:
+    prevnext_enabled_nodetypes:
+      type: sequence
+      label: 'Enabled Node Types'
+      sequence:
+        type: string