Fix issue where RouterBuilder::scope() would not allow 2 arguments.
Rename Route::parseExtensions() to Route::extensions()

Having the same method names across RouteBuilder, and Route make it
easier to remember which method is correct. Additionally, the
RouteCollection will also be getting an extensions method.
Fix issue where Router::extensions() would not be correct.

Due to the way routes are now incrementally built and added to the
collection, Router lost the plot around which extensions existed. By
moving the extension list into the RouteCollection and incrementally
build out the extension list as routes are added we can have the correct
list of extensions. This is important for the few places that
Router::extensions() is still used.
Remove a bunch of dead code.

This code is basically garbage and does nothing.
Add missing type to api docs.
Merge pull request #4345 from cakephp/router-extensions-fix

Router extensions fix