Expose FindRoute on Mux to enable access to it in custom middleware

We are building a custom middleware which needs to know the route that matched the current request path. Unfortunately, the only available option `Match` just returns a yay or nay. Since this method has access to the actual route that matches, can we add a `FindRoute` on `Mux` that will delegate the actual work to `tree.FindRoute` ?

Is there an alternate way to determine the route that matches the current http request path in a custom middleware?

Start by comparing the existing Mux.Match entry point with the tree.FindRoute method mentioned in the issue, then trace how custom middleware accesses the current request path. Done means exposing Mux.FindRoute as a delegation to tree.FindRoute and confirming that middleware can obtain the matching route.

Written by the indexing model from the issue text.