[4.x] Implied Statamic route views

[jasonvarga]

When making simple routes like this:

Route::statamic('login', 'login');

It's a little redundant when the view is the same as the URI. This PR makes the view optional and fall back to use the URI as the view.

Route::statamic('my-page'); // Implies 'my-page'
Route::statamic('/my-page'); // Implies 'my-page'
Route::statamic('/foo/bar'); // Implies 'foo.bar'