Add New Diagram command with starter templates

- Archify: New Diagram… (Explorer folder menu, File > New File…, Command
  Palette) creates a .archify file from a template and opens the diagram
- Six YAML templates: architecture, workflow, sequence, dataflow,
  lifecycle and an everyday plan; connections carry ids, workflows use
  schema 2
- Title and meta.output follow the file name; template comments are kept
- Tests: every template passes archify validate; integration check for
  the command

Closes #2

package.json

@@ -38,6 +38,12 @@
   ],
   "contributes": {
     "commands": [
+      {
+        "command": "archify.newDiagram",
+        "title": "New Diagram\u2026",
+        "category": "Archify",
+        "icon": "$(new-file)"
+      },
       {
         "command": "archify.showPreview",
         "title": "Open Preview",
@@ -145,6 +151,11 @@
         }
       ],
       "explorer/context": [
+        {
+          "command": "archify.newDiagram",
+          "when": "explorerResourceIsFolder",
+          "group": "navigation@10"
+        },
         {
           "command": "archify.showPreview",
           "when": "resourceFilename =~ /(\\.archify|\\.(architecture|workflow|sequence|dataflow|lifecycle)\\.json)$/",
@@ -167,6 +178,12 @@
           "when": "resourceFilename =~ /(\\.archify|\\.(architecture|workflow|sequence|dataflow|lifecycle)\\.json)$/",
           "group": "1_open"
         }
+      ],
+      "file/newFile": [
+        {
+          "command": "archify.newDiagram",
+          "group": "file"
+        }
       ]
     },
     "keybindings": [