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

templates/workflow.archify

@@ -0,0 +1,70 @@
+# Workflow: steps of a process, grouped in lanes, left to right by col.
+# mainPath lists the happy path; edges link node ids.
+schema_version: 2
+diagram_type: workflow
+meta:
+  title: Change Approval
+  output: workflow.html
+  # Rename the legend entries of the node types you use.
+  legend:
+    entries:
+      frontend:
+        label: Author step
+      security:
+        label: Review gate
+      backend:
+        label: Decision
+      external:
+        label: Outcome
+lanes:
+  - id: author
+    label: Author
+  - id: review
+    label: Reviewers
+mainPath: [draft, review, approved, released]
+nodes:
+  - id: draft
+    lane: author
+    col: 0
+    type: frontend
+    label: Draft change
+  - id: review
+    lane: review
+    col: 1
+    type: security
+    label: Review
+    sublabel: two approvals
+  - id: approved
+    lane: review
+    col: 2
+    type: backend
+    label: Approved
+  - id: released
+    lane: author
+    col: 3
+    type: external
+    label: Released
+  - id: rework
+    lane: author
+    col: 1
+    type: frontend
+    label: Rework
+edges:
+  - id: submit
+    from: draft
+    to: review
+    label: submit
+  - id: approve
+    from: review
+    to: approved
+    label: approve
+  - id: release
+    from: approved
+    to: released
+    label: release
+  - id: request-changes
+    from: review
+    to: rework
+    label: changes requested
+    role: branch
+    variant: dashed