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/everyday-plan.archify

@@ -0,0 +1,72 @@
+# Everyday plan: a workflow about daily life rather than software.
+# Node types set the colour; meta.legend renames them for the reader,
+# and icon picks the corner symbol (calendar, clock, person, briefcase, flag, moon).
+schema_version: 2
+diagram_type: workflow
+meta:
+  title: Summer Holiday Plan
+  output: everyday-plan.html
+  legend:
+    entries:
+      backend:
+        label: Time off
+      security:
+        label: Work
+      external:
+        label: Milestone
+lanes:
+  - id: plan
+    label: Plan
+  - id: work
+    label: Work
+mainPath: [today, book, holiday, back]
+nodes:
+  - id: today
+    lane: plan
+    col: 0
+    type: external
+    label: Today
+    icon: clock
+  - id: book
+    lane: plan
+    col: 1
+    type: external
+    label: Book flights
+    sublabel: by end of May
+    icon: flag
+  - id: holiday
+    lane: plan
+    col: 2
+    type: backend
+    label: Holiday
+    sublabel: 2 weeks
+    icon: calendar
+  - id: back
+    lane: plan
+    col: 3
+    type: external
+    label: Back home
+    icon: flag
+  - id: handover
+    lane: work
+    col: 1
+    type: security
+    label: Handover
+    icon: briefcase
+edges:
+  - id: plan-booking
+    from: today
+    to: book
+  - id: go
+    from: book
+    to: holiday
+    label: leave approved
+  - id: return
+    from: holiday
+    to: back
+  - id: prepare
+    from: today
+    to: handover
+    label: before leaving
+    role: branch
+    variant: security