examples: add "cors" demo

examples/workers/cors/cfw.json

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+{
+  "name": "worktop-demo-cors",
+  "zoneid": "<YOUR ZONE ID>",
+  "profile": "personal",
+  "entry": "index.ts",
+  "routes": [
+    "<YOUR DOMAIN>/*"
+  ]
+}

examples/workers/cors/index.ts

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+import { Router, listen } from 'worktop';
+import * as CORS from 'worktop/cors';
+
+const API = new Router();
+
+// TODO: Replace this with persistent layer
+// Example – KV, Durable Object, or third-party
+let counter = 0;
+
+/**
+ * Handles `OPTIONS` requests using the same settings.
+ * NOTE: Call `CORS.preflight` per-route for inidivual settings.
+ */
+API.prepare = CORS.preflight({
+	origin: '*', // allow any `Origin` to connect
+	headers: ['Cache-Control', 'Content-Type', 'X-Count'],
+	methods: ['GET', 'HEAD', 'PUT', 'POST', 'DELETE'],
+});
+
+API.add('GET', '/', (req, res) => {
+	res.setHeader('X-Count', counter);
+	res.send(200, counter);
+});
+
+API.add('POST', '/', (req, res) => {
+	counter += 1;
+	res.setHeader('X-Count', counter);
+	res.send(200, counter);
+});
+
+API.add('PUT', '/', async (req, res) => {
+	try {
+		var next = Number(await req.body<string|number>());
+		if (next * 0 !== 0) return res.send(400, 'Invalid number!');
+	} catch (err) {
+		return res.send(400, 'Error parsing request');
+	}
+	counter = next;
+	res.setHeader('X-Count', counter);
+	res.send(200, counter);
+});
+
+API.add('DELETE', '/', (req, res) => {
+	counter -= 1;
+	res.setHeader('X-Count', counter);
+	res.send(200, counter);
+});
+
+listen(API.run);

examples/workers/cors/readme.md

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+# Example: CORS
+
+> [View in Playground](https://cloudflareworkers.com/#955c634d65e1ebcb97be610f087be973:https://tutorial.cloudflareworkers.com/)
+
+Configures [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) headers for multiple endpoints in this example:
+
+* `GET /` – displays the current `counter` value
+* `POST /` – increment the `counter` & return its new value
+* `PUT /` – set the `counter` value (via request body) & return its new value
+* `DELETE /` - decrement the `counter` & return its new value
+
+## License
+
+MIT