Model page: Registry panel lists registry files, elements inside each

- one collapsible header per .archify-registry file with its element
  count and an Open button that opens the registry view; files start
  collapsed, the Registry panel open
- view types moved to views/viewTypes.ts so the diagram page can open a
  registry without a circular import
- 0.7.1

src/views/viewTypes.ts

@@ -0,0 +1,6 @@
+/** Custom editor view types, in one place so views can refer to each other without importing each other. */
+export const PREVIEW_VIEW_TYPE = 'archify.preview';
+/** Default editor for .archify files: the rendered diagram. */
+export const DIAGRAM_EDITOR_VIEW_TYPE = 'archify.diagram';
+/** Default editor for .archify-registry files: the elements as cards. */
+export const REGISTRY_VIEW_TYPE = 'archify.registry';