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/0100-context/web-app.archify

@@ -0,0 +1,26 @@
+schema_version: 1
+diagram_type: architecture
+meta:
+  title: System context
+  output: context.html
+components:
+  - ref: user
+    pos: [40, 60]
+    size: [130, 60]
+  - ref: admin
+    pos: [40, 260]
+    size: [130, 60]
+  - ref: webapp
+    pos: [280, 160]
+    size: [160, 60]
+  - ref: idp
+    pos: [530, 60]
+    size: [150, 60]
+  - ref: email
+    pos: [530, 260]
+    size: [150, 60]
+connections:
+  - { from: user, to: webapp, label: uses, variant: emphasis }
+  - { from: admin, to: webapp, label: administers }
+  - { from: webapp, to: idp, label: signs in via, variant: security }
+  - { from: webapp, to: email, label: sends mail }