Add archify-vscode-ext skill: Archify diagrams and models for the Archify Diagram Viewer extension

Model tasks (validate, list, scaffold, resolve), diagram tasks (validate,
render), the bundled Archify 3.0.1 runtime and yaml parser under
scripts/vendor, references for authoring, formats and folders, and the
online-shop example model.

scripts/vendor/yaml/schema/common/null.js

@@ -0,0 +1,15 @@
+import { Scalar } from '../../nodes/Scalar.js';
+
+const nullTag = {
+    identify: value => value == null,
+    createNode: () => new Scalar(null),
+    default: true,
+    tag: 'tag:yaml.org,2002:null',
+    test: /^(?:~|[Nn]ull|NULL)?$/,
+    resolve: () => new Scalar(null),
+    stringify: ({ source }, ctx) => typeof source === 'string' && nullTag.test.test(source)
+        ? source
+        : ctx.options.nullStr
+};
+
+export { nullTag };