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

package.json

@@ -51,6 +51,11 @@
         "title": "Archify | New Diagram…",
         "icon": "$(new-file)"
       },
+      {
+        "command": "archify.newModel",
+        "title": "Archify | New Model…",
+        "icon": "$(new-folder)"
+      },
       {
         "command": "archify.showPreview",
         "title": "Archify | Open Preview",
@@ -171,19 +176,29 @@
           "group": "zzz_archify@1"
         },
         {
-          "command": "archify.showPreview",
-          "when": "resourceFilename =~ /(\\.archify|\\.(architecture|workflow|sequence|dataflow|lifecycle)\\.json)$/",
+          "command": "archify.newModel",
+          "when": "explorerResourceIsFolder",
           "group": "zzz_archify@2"
         },
         {
-          "command": "archify.renderToHtml",
+          "command": "archify.showPreview",
           "when": "resourceFilename =~ /(\\.archify|\\.(architecture|workflow|sequence|dataflow|lifecycle)\\.json)$/",
           "group": "zzz_archify@3"
         },
+        {
+          "command": "archify.renderToHtml",
+          "when": "resourceFilename =~ /(\\.archify|\\.(architecture|workflow|sequence|dataflow|lifecycle)\\.json)$/",
+          "group": "zzz_archify@4"
+        },
         {
           "command": "archify.openHtmlViewer",
           "when": "resourceExtname == .html",
-          "group": "zzz_archify@2"
+          "group": "zzz_archify@3"
+        },
+        {
+          "command": "archify.openModel",
+          "when": "resourceFilename =~ /\\.archify(-registry)?$/",
+          "group": "zzz_archify@5"
         }
       ],
       "editor/title/context": [
@@ -197,6 +212,10 @@
         {
           "command": "archify.newDiagram",
           "group": "file"
+        },
+        {
+          "command": "archify.newModel",
+          "group": "file"
         }
       ]
     },