feat(16.2-01): host plugin field type elements in the admin SPA

- loadFieldTypes fetches GET /field-types after sign-in, loads each script
  once and adds the styles as global links activateStyles never disables
- PluginField creates the type's element, sets the value property and the
  contract attributes, takes summer-change as the form value and never
  writes back a value the element just emitted
- rendererForField and isRegisteredField route any field with an element
  to PluginField in forms, previews and the save payload
- rebuilt modules/boardwalk/dist

admin/src/components/form/formContext.ts

@@ -92,5 +92,11 @@ export const WIDGET_EVENT = 'summer-action'
  */
 export const WIDGET_RESULT_EVENT = 'summer-result'
 
+/**
+ * Event a plugin field type's element dispatches (bubbling) when the
+ * administrator edits its value; detail.value is the new form value.
+ */
+export const FIELD_CHANGE_EVENT = 'summer-change'
+
 /** How long a widget waits for its custom element to be defined. */
 export const WIDGET_TIMEOUT = 5000