feat: add endpoint router wildcard

[esbenbjerre]

Description

This PR adds a wildcard handler that translates to a catch-all route template. The purpose is to have a simple, readable way of expressing a default handler.

How to test

let endpoints =
  [
    GET [
      route "/a" a
      route "/b" b
    ]
    wildcard notFound
  ]

where a, b and notFound are appropriate HttpHandlers.

Related issues

#534
#537