feat: enable adding types for Env variables

[yusukebe]

Enable adding types for Env variables using generics in Module Worker mode.

interface Env {
  API_KEY: string
  API_SECRET: string
  MY_KV: KVNamespace
}

const app = new Hono<Env>()

app.get('/', (c) => {
  const key = c.env.API_KEY
  const secret = c.env.API_SECRET
  const message = getMessage({ key, secret })
  return c.text(`Hono! ${message}`)
})

export default app