feat(net): getAvailablePort() (#3890)

Co-authored-by: Yoshiya Hinosawa <stibium121@gmail.com>
Co-authored-by: Asher Gomez <ashersaupingomez@gmail.com>

net/get_available_port.ts

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+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+
+/**
+ * Returns an available network port.
+ *
+ * @example
+ * ```ts
+ * import { getAvailablePort } from "https://deno.land/std@$STD_VERSION/net/get_available_port.ts";
+ *
+ * const port = getAvailablePort();
+ * Deno.serve({ port }, () => new Response("Hello, world!"));
+ * ```
+ */
+export function getAvailablePort(): number {
+  const listener = Deno.listen({ port: 0 });
+  listener.close();
+  return (listener.addr as Deno.NetAddr).port;
+}

net/get_available_port_test.ts

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+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+
+import { getAvailablePort } from "./get_available_port.ts";
+import { assertEquals } from "../assert/mod.ts";
+
+Deno.test("getAvailablePort() gets an available port", async () => {
+  const port = getAvailablePort();
+
+  const server = Deno.serve({
+    port,
+    async onListen() {
+      const resp = await fetch(`http://localhost:${port}`);
+      const text = await resp.text();
+      assertEquals(text, "hello");
+      server.shutdown();
+    },
+  }, () => new Response("hello"));
+
+  await server.finished;
+});

net/mod.ts

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+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+
+/**
+ * Network utilities.
+ *
+ * @module
+ */
+
+export * from "./get_available_port.ts";