Add missing ui_patterns_source.icon config schema.

Closes #3493153

modules/ui_icons_patterns/config/schema/ui_icons_patterns.schema.yml

+ui_patterns_source.icon:
+  type: mapping
+  label: 'Source: Icon'
+  mapping:
+    value:
+      type: mapping
+      nullable: true
+      label: 'Default icon value'
+      mapping:
+        target_id:
+          type: string
+          label: 'Default icon full ID'
+        settings:
+          type: sequence
+          label: 'Default icon settings'
+          sequence:
+            type: ui_icons.icon_pack_options.[%key]

[Hervé Donner]

Not 100% sure about this but the value can be NULL since \Drupal\ui_icons_patterns\Plugin\UiPatterns\Source\IconSource::getPropValue can return NULL. Maybe it should return an array instead?

[Jean Valverde]

It's not clear for me on the UI patterns side, sources do as they want and getPropValue() is called only in Drupal\ui_patterns\SourcePluginBase::getValue() which when NULL seems to be set as empty array when called elsewhere. So getPropValue can return array in all cases.

[Hervé Donner]

Copy pasted from field.value.ui_icon.
Should we centralize this definition in ui_icons and use it for both field.value.ui_icon and ui_patterns_source.icon to avoid duplication?