feat(serve): add --unix-socket flag

[tarasglek]

Closes #26160
Note this is coded with an llm. I checked the code and that it works.

Implements unix socket support for deno serve via --unix-socket flag.

Implementation

• Adds --unix-socket CLI flag in `cli/args/flags.rs` (conflicts with --port/--host)
• Sets DENO_SERVE_ADDRESS=unix:<path> env var in `cli/tools/serve.rs` before worker starts
• Runtime picks this up via existing `Deno.serve` env var handling

Test

echo "export default { fetch: () => new Response(\"ok\") }" > server.ts
deno serve --allow-all --unix-socket /tmp/deno.sock server.ts &
curl --unix-socket /tmp/deno.sock http://localhost/
# ok

[CLAassistant]

All committers have signed the CLA.