Issue #3393163 by p-neyens: provide config schema

Closes #3393163

config/schema/layoutbuilder_extras.schema.yml

 layoutbuilder_extras.settings:
   type: config_object
   label: 'Layoutbuilder extras'
+  mapping:
+    section_actions_position:
+      type: string
+      label: 'Position of the section actions'
+      constraints:
+        Choice:
+          - left
+          - top
+    enable_redirect_on_save:
+      type: boolean
+      label: 'Enable redirect to layout builder on node save'
+    enable_configure_ajax_save:
+      type: boolean
+      label: 'Enable layout builder configure form live changes'
+    remove_empty_divs:
+      type: boolean
+      label: 'Remove empty divs'
+    enable_drag_handle_icon:
+      type: boolean
+      label: 'Enable drag handle icon'
+    enable_admin_css:
+      type: boolean
+      label: 'Enable admin css'
+    contextual_links_only_lb:
+      type: boolean
+      label: 'Only allow contextual links on layout builder'
+    contextual_links_roles:
+      type: sequence
+      label: 'User roles'
+      sequence:
+        type: string
+        label: 'Role'