perf(router): avoid Promise allocation in synchronous dispatch path

[cesco69]

Add sync-first dispatch via _syncDispatch/_syncNext that skips new Promise() when route callbacks are synchronous.
Falls back to _routeRequestAsync only for Router instances or async callbacks.

Eg:

app.use(cors());        // sync -> no Promise allocated
app.use(helmet());      // sync -> no Promise allocated

app.get('/data', async (req, res) => {
  const rows = await db.query('SELECT 1');
  res.json(rows);
});
// ^ async -> falls back to Promise only from here