Add support for r.PathValue when built for Go 1.22.

Go 1.22 introduced pattern matching into its ServeMux, and as a result
the net/http.Request type got a PathValue method, returning the value
that matched the given key in the pattern.

We can, and should, populate the data behind that mathod when using
trout's pattern matching, so handlers can standardize on that built-in
function.

route.go

@@ -232,6 +232,9 @@ func (router Router) getHandler(r *http.Request) http.Handler {
 	r.Header.Set("Trout-Pattern", route.pattern)
 	for key, vals := range route.params {
 		r.Header[http.CanonicalHeaderKey("Trout-Param-"+key)] = vals
+		for _, val := range vals {
+			setBuiltinRequestPathVar(r, key, val)
+		}
 	}
 
 	// if no handler is set, it could be because there's no handler for

route_below_go1_22.go

@@ -0,0 +1,8 @@
+//go:build !go1.22
+
+package trout
+
+import "net/http"
+
+func setBuiltinRequestPathVar(_ *http.Request, _, _ string) {
+}

route_go1_22.go

@@ -0,0 +1,9 @@
+//go:build go1.22
+
+package trout
+
+import "net/http"
+
+func setBuiltinRequestPathVar(r *http.Request, name, value string) {
+	r.SetPathValue(name, value)
+}

route_go1_22_examples_test.go

@@ -0,0 +1,31 @@
+//go:build go1.22
+
+package trout_test
+
+import (
+	"net/http"
+
+	"darlinggo.co/trout/v2"
+)
+
+func ExampleRouter_Endpoint_pathValues() {
+	postsHandler := http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			// we populate the PathValue request property introduced
+			// in Go 1.22 when you build with Go 1.22.
+			id := r.PathValue("id")
+			_, err := w.Write([]byte(id))
+			if err != nil {
+				panic(err)
+			}
+		})
+
+	var router trout.Router
+	router.Endpoint("/posts/{id}").Handler(postsHandler)
+
+	req, _ := http.NewRequest("GET", "http://example.com/posts/foo", nil)
+	router.ServeHTTP(exampleResponseWriter{}, req)
+
+	// Output:
+	// foo
+}