Clarify plugin authoring and external dev workflow

packages/plugins/examples/plugin-authoring-smoke-example/vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    include: ["tests/**/*.spec.ts"],
+    environment: "node",
+  },
+});