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/dataflow.archify

@@ -0,0 +1,55 @@
+# Data flow: where data comes from, how it is transformed, and where it ends up.
+# Nodes sit in stages (columns) and rows; flows link node ids.
+schema_version: 1
+diagram_type: dataflow
+meta:
+  title: Sales Reporting
+  output: dataflow.html
+  viewBox: [960, 420]
+stages:
+  - label: Sources
+  - label: Processing
+  - label: Storage
+  - label: Consumers
+nodes:
+  - id: shop
+    type: frontend
+    label: Web Shop
+    stage: 0
+    row: 0
+  - id: crm
+    type: external
+    label: CRM
+    stage: 0
+    row: 1
+  - id: etl
+    type: backend
+    label: Nightly ETL
+    stage: 1
+    row: 0
+  - id: warehouse
+    type: database
+    label: Warehouse
+    stage: 2
+    row: 0
+  - id: dashboard
+    type: frontend
+    label: Sales Dashboard
+    stage: 3
+    row: 0
+flows:
+  - from: shop
+    to: etl
+    label: orders
+  - from: crm
+    to: etl
+    label: customers
+    classification: PII
+    variant: security
+  - from: etl
+    to: warehouse
+    label: daily facts
+    variant: emphasis
+  - from: warehouse
+    to: dashboard
+    label: aggregates