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/people.archify-registry

@@ -0,0 +1,12 @@
+schema_version: 1
+elements:
+  user:
+    type: external
+    label: User
+    sublabel: browser
+    description: Anyone with an account; uses the application from a browser.
+  admin:
+    type: external
+    label: Administrator
+    sublabel: back office
+    description: Manages users and content from the same application with extra rights.