Models: main.archify, registries, refs, links and the model page; 0.7.0

A folder with main.archify at its top is a model, a set of diagrams
that belong together. Opening main.archify shows the model page: the
model's name and description, its own overview diagram when the file
has one, the diagrams grouped by folder (click to open), and the
registry with where each element is used. The page is built by
traversing the folder tree and refreshes when files change.

*.archify-registry files define shared elements once; diagrams draw
them with `ref: <id>` plus position and may override label, sublabel
and tag. References are resolved before Archify runs, so Archify is
unchanged. Hover, go-to-definition and completion on ref; unknown
names, forbidden overrides, duplicate ids and broken registries in
Problems; registry edits re-render open diagrams. `diagram:` links on
elements and components open the linked diagram on double-click.

New command Archify | Open Model, a Model template for New Diagram…,
editor schemas for the new keys and for registries, docs/model.md and
examples/online-shop. Single diagrams behave exactly as before.

templates/registry.archify-registry

@@ -0,0 +1,18 @@
+# Shared elements of the model: one name for one thing. A diagram refers to
+# an element with `ref: <id>` and adds only its position and size.
+schema_version: 1
+elements:
+  api:
+    type: backend
+    label: API
+    sublabel: REST service
+    description: >
+      What this thing is and does, in one place. Shown on hover and on the
+      model page wherever the element is used.
+    # diagram: 0300-components/api.archify   # the diagram that explains it
+  db:
+    type: database
+    label: Database
+    sublabel: PostgreSQL
+    brand: postgresql
+    description: The source of truth.