Add Archify | New Model… with three C4 examples and Open Model in the Explorer menu

- templates/models: C4 - Standard Web App, C4 - E-Shop, C4 - Event-Based
  Integration; each a complete model (main.archify, registry, context,
  containers, components) that validates
- archify.newModel: pick an example and a folder name, copy it, name the
  model after the folder, open the model page
- Explorer menu: New Model… on folders, Open Model on .archify and
  .archify-registry files; File → New File… lists New Model…
- unit test validates every example end to end; integration test runs the
  command; README, CHANGELOG, docs/model.md

templates/models/c4-standard-web-app/registry/systems.archify-registry

@@ -0,0 +1,18 @@
+schema_version: 1
+elements:
+  webapp:
+    type: cloud
+    label: Web Application
+    sublabel: the system
+    description: Everything we build and run; the single-page app, the API and its stores.
+    diagram: 0200-containers/web-app.archify
+  idp:
+    type: external
+    label: Identity provider
+    sublabel: OpenID Connect
+    description: Signs users in and issues tokens; we never store passwords.
+  email:
+    type: external
+    label: Email service
+    sublabel: transactional mail
+    description: Sends sign-up confirmations and notifications.