Add a Deno demo for fetch-router

[kuboon]

A router created with createRouter() already exposes a fetch method, which is exactly the shape Deno.serve and deno serve expect. I have been running Remix 3 on Deno for a while now and it has been a really pleasant fit, so I would like to add a Deno demo next to the existing Bun and Node ones under packages/fetch-router/demos/.

It is the same blog application as the Node and Bun demos, so the three sit side by side and show that the router, middleware, and route helpers are runtime-agnostic. The difference is that on Deno there is no server entry point to write at all: deno serve does the rest.

• Adds packages/fetch-router/demos/deno/ with the same routes, actions, and in-memory data as the Bun demo
• The router is the default export, so deno serve picks up its fetch method and Deno owns the listener, the port, and graceful shutdown on SIGINT/SIGTERM
• Serves public/ with staticFiles() from @remix-run/static-middleware instead of the hand-rolled file handler in the Bun demo, which also exercises Deno's node:fs/node:url compatibility
• deno.json sets "nodeModulesDir": "manual" so Deno resolves @remix-run/* from the pnpm workspace's node_modules, and the tasks run with least-privilege permissions on port 44100

// deno.json
{
  "tasks": {
    "dev": "deno serve --watch --allow-read=./public --allow-env=CI,FORCE_COLOR,NO_COLOR,TERM --port 44100 app/router.ts",
    "start": "deno serve --allow-read=./public --allow-env=CI,FORCE_COLOR,NO_COLOR,TERM --port 44100 app/router.ts",
    "typecheck": "deno check"
  },
  "nodeModulesDir": "manual"
}

A few notes for reviewers:

The demo intentionally defines no typecheck or test script in its package.json, so pnpm -r typecheck and pnpm test stay green on machines without Deno installed. This matches the existing packages/multipart-parser/demos/deno. Type checking is available through deno task typecheck.
@types/node is a dev dependency because Deno needs it present in node_modules to resolve the node type reference that comes in through the @remix-run/* sources.
The permissions are deliberately narrow: static imports need no read permission, so --allow-read=./public covers the only runtime filesystem access, and --allow-env is limited to the four variables logger() inspects for color detection through @remix-run/terminal. deno serve provides its own network access, so no --allow-net is needed.
Tested with Deno 2.9.4.