[5.x] Auto-load addon routes

[duncanmcclean]

It's pretty common for an addon to have routes/web.php, routes/cp.php, routes/actions.php route files. Right now, you have to configure these yourself in the addon's ServiceProvider:

protected $routes = [
    'web' => __DIR__.'/../routes/web.php',
    'cp' => __DIR__.'/../routes/cp.php',
    'actions' => __DIR__.'/../routes/actions.php',
];

However, if you're following convention, it would be nice if Statamic would automatically register an addon's route files, in a similar fashion to how fieldtypes, tags, etc are autoloaded.

Inspired by #9270.