v4: Add interaction router

[diamondburned]

This issue proposes a new package utils/interactions which supplies an interactions.Router that the user can use. The router should be used vaguely like so:

r := interactions.NewRouter() // composes CommandRouter
r.Use(interactions.WithDeferAfter(2 * time.Second)) // automatically defer responses
r.Modal("verify-response", func(data *interactions.ModalEvent) {
    time.Sleep(5 * time.Second)
    data.RespondMessage(api.InteractionResponseData{ /* TODO */ })
})
r.Button("button-press", func(press *interactions.ButtonEvent) {
    press.RespondMessage(...)
})
r.Subcommand("member", func(r *interactions.CommandRouter) {
    r.Command("query", h.queryMember)
})
r.Command("query-member", h.queryMember)

// TODO: API draft