feat: Add `deno serve` subcommand (#23511)

By default, `deno serve` will assign port 8000 (like `Deno.serve`).
Users may choose a different port using `--port`.

`deno serve /tmp/file.ts`

`server.ts`:
```ts
export default {
  fetch(req) {
    return new Response("hello world!\n");
  },
};
```

3 files changed, 10 insertions, 0 deletions

diff --git a/tests/specs/serve/bad/__test__.jsonc b/tests/specs/serve/bad/__test__.jsonc
new file mode 100644
index 000000000..9a37d60ff
--- /dev/null
+++ b/tests/specs/serve/bad/__test__.jsonc
@@ -0,0 +1,5 @@
+{
+  "args": "serve --port 12345 main.ts",
+  "output": "main.out",
+  "tempDir": true
+}
diff --git a/tests/specs/serve/bad/main.out b/tests/specs/serve/bad/main.out
new file mode 100644
index 000000000..1bb42f909
--- /dev/null
+++ b/tests/specs/serve/bad/main.out
@@ -0,0 +1 @@
+error: deno serve requires export default { fetch } in the main module, did you mean to run "deno run"?
diff --git a/tests/specs/serve/bad/main.ts b/tests/specs/serve/bad/main.ts
new file mode 100644
index 000000000..3399721d4
--- /dev/null
+++ b/tests/specs/serve/bad/main.ts
@@ -0,0 +1,4 @@
+export default {
+  bad() {
+  },
+};