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/sequence.archify

@@ -0,0 +1,50 @@
+# Sequence: who calls whom, in order. Participants are columns;
+# messages run top to bottom by y.
+schema_version: 1
+diagram_type: sequence
+meta:
+  title: Login Request
+  output: sequence.html
+  viewBox: [960, 520]
+participants:
+  - id: client
+    type: external
+    label: Client
+  - id: api
+    type: backend
+    label: API
+  - id: auth
+    type: security
+    label: Auth Service
+  - id: db
+    type: database
+    label: User Store
+messages:
+  - from: client
+    to: api
+    y: 160
+    label: POST /login
+    variant: emphasis
+  - from: api
+    to: auth
+    y: 210
+    label: verify credentials
+  - from: auth
+    to: db
+    y: 260
+    label: find user
+  - from: db
+    to: auth
+    y: 310
+    label: user record
+    variant: return
+  - from: auth
+    to: api
+    y: 360
+    label: token
+    variant: return
+  - from: api
+    to: client
+    y: 410
+    label: 200 OK + token
+    variant: return