Support http.Request.PathValue

# Description
Support [Request.PathValue](https://pkg.go.dev/net/http#Request.PathValue) for compatibility.

# Describe the feature you'd like to request
cf. https://github.com/go-chi/chi/pull/901
cf. https://github.com/go-chi/chi/issues/873

# Describe the solution you'd like
cf. https://github.com/go-chi/chi/pull/901
cf. https://github.com/go-chi/chi/issues/873