Feature: reverse URL creation

[mindplay-dk]

Add support for reverse URL creation, e.g. for the following route:

$router->route('users/<username:slug>')->get(ShowUser::class);

This feature would enable URL creation based on the defined route template, e.g.:

$user_url = $url_helper->createURL(ShowUser::class, ["username" => $username]);

Where the Router currently maintains only a forward map from route-pattern to controller/action-name, with this feature, it would (internally?) maintain a reverse map of controller/action-name to route-pattern.

The ability to perform a reverse look-up enables us to template the URL patterns and create URLs, which either (A) eliminates maintenance of synchronous URL creation (matching defined routes) in hand-written URL helpers, or (B) eliminates hand-written URL helpers entirely, depending on developer preference.

TBD: Milestone 2.x or 3.0?