Allow user to opt-out from static serving (sirv)

[brillout]

Currently, vike-node uses sirv to serve static assets. We could/should allow users to bring their own static middleware.

With @magne4000 we have plans to allow the user to completely own middleware installation (so that the user can fully determine middleware order). With that plan, I guess we can simply tell users to skip adding the static middleware.

[magne4000]

@universal-middleware/sirv now exists and is a step in the right direction to be able to implement this.